As well as fitting winglets, Al has a number of other modifications and simple improvements that can be done to the 'Mini' to help improve its performance. Details of these are given extensively below.

1. FLAP TORQUE TUBES...seal any way you like but try an automobile CV boot with a hole roughly the size of the torque tube.  We're talking abut sealing that whole wing fuselage juncture. 
 
2.  Replace or fix up the CONTROL ROD SEALS on the wing root.  If factory sealing material is not available, consider large beach balls, cutting off a foot or so below the neck and cut and paste it to your satisfaction.
 
3. CANOPY FIT;  When doing the fuselage, check the canopy for exact fit.  I used a filler in some places and a pneumatic sander in others.  Many older Minis have had the wind catch the canopy and the hinges have been broken.  This is a good time to really make it flush and get the fit right.  Once the canopy sill is gelcoated, the fit is completely different as you might know.
 
4. CG TOST HOOK SEALING;  When you have the seat and seat back stuff out, check or replace all seals around the CG hook.  That includes the SS cable going forward .  While you are there, check the records to see if it has ever been replaced as required by the TOST folks.  At the minimum, blow out all the dirt and dust that accumulates.  I had a 200 ft inadvertant release after takeoff just over the lake water at Jimmy Stoia's field near Manning SC.  It was dirt and dust in the mechanism after I had refinished the fuselage.  Yup I made a 180 and had to pull on airbrakes to land downwind. 
 
5.  Install a PEE TUBE for long flights.  If you're up tight, you can't make right decisions X-C.  One hole is drilled in the seat pan behind the stick and one hole is drilled in the right rear of the wheel well.  Use hard somewhat hard,flexible tubing under the seat with fittings for hospital/surgical tubing on both ends.  Buy a soaring season supply of Male External Catheters from any Medical Supply Store.  Secure the tubing with nylon ties once it goes on the starboard side (rear) of the wheel well cover.  The 90 degree fitting from the semi hard tubing is for the actual drain tube which you  secure to the right wheel door.  From the 90 degree fitting, it should go straight down as close to the bottom edge of the door as possible.  You too can sit in the cockpit on the grid with the rest of them with a small towel over your crotch.  Keeps the sun off don't you know.  You gotta figure out for yourself where you are going to put the catheter on prior to take off.  While in the general gear well area, put a piece of mylar on one gear door for seal.  Check for fit. 
 
6.  I put on WINGLETS.  I bought a reasonably priced kit from M&H up in El Mira, Corning, NY area from Heinz.  He might still sell a kit which consists of two carbon fiber shells (with the attach angle accurately cut at the factory), aircraft grade plywood rough cut ribs, various hard to get fiberglass tubing and rods etc.  He does not sell the guts it takes to take a hand saw and cut your tips off just outboard of the ailerons.  Heinz walked me though the steps and it was basically painless.  Lots of filler and resin glue are needed.  Mine had ASW-20 airfoil shapes to the 'root' of the winglets so lots of filling and sanding was required to get everything just right.  Result:  Aileron response greatly improved plus an unknown LD increase but I think you have read of the winglet reports by masak and johnson, etc.  Reducing that humongus low speed wing tip vortex swirl over the top of the wing tip (with winglets) gets the air flowing the right way on the outer aileron and improves lift.  This translates to better L/D in the 40 to 55kt area based on all winglet testing I have read.  Most wings with winglets have benefit crossover at about 80Kts.  All my performance testing in my Mini has been without winglets.
 
7.  REAR VENT:  The rear vent I installed relieves the air pressure buildup from the nose vent that wants to go out into the wing and exit over the control surfaces.  With side vent closed, ventilation could be better. The same wing root offending air comes in for ventilation but most of it goes out the rear vent when modified.  The vent is a NACA vent or NASA Vent which is normally seen on commercial and military aircraft.  With the point going forward, it sucks air in.  With the point going aft, it has a reciprocal function and vents the air out just ahead of the tail wheel you will want to put on.  T. Knauff sells the tail wheels.  Do a careful CG measurement iaw the handbook formulas. Required after major repair.  I recommend 2/3 aft from datum and not way aft as some like.  I mention this as now is the time to put 1/8 inch sheet lead around and above the tail wheel to get the CG in the right place.  I know, tail wheels are double stick glued on (to come off under side loads) to prevent damage in a ground loop!!!  Fill and smooth around the lead when you have the CG in the 'ideal' location for you. (with chute if you wear one).  Again this requires a cut into the underfuselage, which requires internal strengthening which requires hardware cloth chicken wire type stuff to be fitted into place to keep the vermin etc, out.   Use the same surface refinishing that Applebay does.  I would avoid mixing different kinds of resurfacing materials.  He can advise you on how he actually does it I'm sure.  Follow his advice.
 
After transition/checkout and depending on your experience, try test flights with measured weight 1/8 in lead taped on with duct tape above the tailwheel /skid. (IAW CG calculations).  With CG too far aft, a quick check is that the 1G stalls will fall off on either wing rather consistently.  With 2/3 aft CG, the nose will fall through slightly forward without the drastic wing drops associated with the too far aft CG stalls.  It also means that with too far AFT cg, that spin entry is easier on those cross-controlled final overshoots that none of us pilots ever do. BTW, don't be jocking the airbrakes all around base and final with the Mini.  +8deg flaps on downwind along with gear opposite touchdown point, and then full airbrakes as required on final to make a minimum energy landing.  Don't jocky the airbrakes. On calm wind takeoffs, follow the handbook exactly.....aerotow or auto/winch.
 
I space my downwinds 45 degrees down toward touchdown while I am on downwind. That's close in.  Fly 55 kts in calm air around base and final.  As I said, I recommend full airbrakes to a min energy stick in the gut touchdown.  If you have been used to wheelin'' in your landings, this will take some getting used to.  Get an IP to help you out if necessary.  Some folks fight a bounced  wheel landing and get into PIOs.  Learning to min energy land reduces that PIO stuff.  Off field landings are safest when you touchdown at min energy and stop in the least amount of rollout.
 
8.  CG ADJUSTMENT on TOP of the vertical stabilizer:  When you carry water, (I like about 4 or 5 gallons each side to get rid of that cork in the ocean feeling on a strong day), the CG moves forward.  The area just below where the horizontal stab sits and on top of the vertical stab, there is a small place for a small amount of sheet lead.  Do your comps on paper and figure out how much weight in addition to the tail wheel weight, is needed up top to stay in a safe/ideal CG area.  Trim and secure the sheet lead with vertical bolts into the top 'rib'.  When dry, take the weight out.  Schempp Hirth makes a very strong tail plus that tail cone is filled with strong braces.  The bird flys well at 2/3 or so aft of datum (LE at wing root). That's both running and climbing.
 
9.  STATIC VENTS:  Always use the under wing static vents for the airspeed.  Otherwise the factory CAL airspeed tables will be off.  The Altimeter statics are on either side of the instrument panel.  When the big TE boxes and fuselage venturies were in vogue, the fuselage boom vents were also used.  When finishing the fuselage, protect these vents with 'headed' pins and/or tape to keep the dirt out.  The TE tube needs protection too of course.  Just before putting in the instrument panel, check all lines for leaks (pressure or suction) WITHOUT instruments attached and vents taped over.  When you are in the cockpit, I suggest sprucing things up a bit and painting most everything Schempp Hirth gray.  I painted mine two tone gray  and the seat covers a black and white stripe which looks OK.  On the inside sides of the fuselage, I glued in a gray padded  material that adds a bit to the cockpit. That original factory interior was gaudy.  I was always used to a shorter control stick in the fighters I flew so I cut an inch off the top of the Mini's stick and put the formfitting handgrip back on no problem.  You will probably have to fit a press to talk switch in the stick anyhow to talk on the radio. 
 
10. The wings will have full mylar and internal seals replaced/installed. I like both internal cloth and external mylar seals.  The original factory wing gap cloth tape was terrible.  I put the large zig zag turbulator tape on the bottom of each wing full length at about .70 chord.  Others suggest .66 and some .75 chord. I have even heard that none is required.  I like .70 chord placement. Gluing metal vortex generators at the top wing/ fuselage juncture cuts a big separation bubble down to something manageable.  Rubber cement some yarn tufts in that area and have someone watch them stand up on end at low thermal speeds.....that means a big separation bubble on top, near the fuselage.  Vortex generators at max chord height out to a foot and a half  from the fuselage help lay those yarn tufts straight aft.  Fine tuning a 78 bird requires us to pay attention to every small air leak, every little bump on the airfoil and wing root seals.  When you have all the basic stuff done, and you are comfortable with all the handbook instructions on the critical take off and landing part of the operation, you will want to make a 700 foot (safe) high speed pass over the airfield with a couple of pilots on the ground listening for whistles, rumbles, any noise whatsoever.  Check to see on a pull up if your gear doors open.  If so, increase the spring tension on each door.  Any noise that the ground pilots hear is fixable.  NOISE translates to DRAG.  No reason why a great bird like the Mini should not out climb and outrun a good many other 15M newer birds when we pay attention to the details. Many new birds are not sealed properly. Listen to them go by on a contest finish.  Some are really noisy.