You could take the carpet up, remove the heavy sound deadening, put the carpet back, so that you have something useable for daily use. Then on trackdays, just remove the spare wheel (you'll probably want to take a full spare set of wheels/tyres to a track day anyway) and take out the rear seat.
Also make sure you don't fit any ICE. If you need sounds just leave it as a headunit connected to standard sized speakers in the dash/front doors and remove the speakers from the rear doors.
You could also convert all your windows to manual winders. That way you'll save Kilos of weight on the window motors.
Plastic windows is a good shout, perhaps just the rears and rear hatch screen (if you don't mind losing the demister).
