Help required!!

Advice required - just purchased a '99 S1, for use only on the track and getting to and from the track. The car is fitted with a sports exhaust and a cat by-pass. Would appreciate any thoughts on ‘must’ up-grades. Had in mind brakes, induction kits, suspension, wheel and tyre combinations and short shift (seems to be a lot of different ones available, from Motor Build to B&M?. I would appreciate your comments, chaps.

hello mate… got myself a 99T mark 1…ive already had a cat replacement pipe put on the vehicle and have recently just fitted a sports exhaust to it which is un believealby loud…for every day use, im thinking of getting the cats put back in do you think this will make sufficent difference… because i cant put up with that from day too day im only a young lad and the police will have me off the road in no time??