My first recommendation would be suspension, ROW30 or a nice set of coilovers will lower the car, firm up the ride and most importantly greatly reduce body roll on hard cornering, makes the car much more fun to drive.
Weight-loss is also good, thre first one is very cost effective*, the others are not, but all save weight:
Braille lightweight battery 10-15 lbs total weight*
Lightweight wheels
Lightweight muffler and/or exhaust system
GT3 or similar lightweight seats
Lightweight flywheel (if you are replacing clutch anyway)
Ed
