How dedicated a track day car are you after?
If you want a road registered race car you might be better of just buying a race prepared car in the first place. Lodge Sports quoted me £10K for a full race spec 944 prepared for the, then fledgling, 944 series; this was about five years ago though.
I do get the impression there will be a move to "outlaw" race cars using track days as a cheap test day, so this may work against you.
The main thing is to make sure that the car is in a good state of repair mechanically. Once that is achieved, if you want to go further:-
The S2 is limitted in how much extra power you can extract, so any significant improvement is only going to come through sheding weight and improving grip/ braking.
A good set of track day wheels and super grippy tyres would help grip and also save money (in the long run) by avoiding accellerated ware of you road tyres. Get 17" wheels for this as 16" slicks are like rocking horse poo.
As I'm a man who likes to fry brakes (it would seem) I'd go for heftier stoppers as bleeding the brakes at lunch time is such a fag. Fit late Turbo front struts so that you can then fit the 250 Turbo/928 S4 "medium" [
] black callipers and discs or fit radial mount addapters to take the same brakes - mind as the back of the thicker disc is very close to the steering ball joint. You could go big reds/black of course but then you are into big money.
Use a good brake fluid and good pads.
Lower and stiffer front springs look good [
] while you have the front struts off, and a strut brace may help tighten the front up a bit. And why not sitck on a set of 968 M030 anti roll bars while you are about it. 30mm dia front and adjustable rear - Mmmm Nice!
Paul knows the weight of everything within the car so he can tell you what to pull out, but spare wheel and rear seats are a good starting place. The front seats are very heavy and you could loose a good bit of weight by swapping them for something a bit more racey.