Hi,
Your best bet would be a set of 15" mk2 alloys with 195/50/15's. If you get the usual VW cross spokes that were fitted at that time, how would the insurance company ever know they weren't standard fit anyway?

For reference the usual tyre sizes used on a mk2, in case you want to go mad are:
195/50/15
205/45/16 or 195/45/16
205/40/17
Just moving to 15's you will notice the car handles better

A performance filter by itself, even an induction kit is never going to manage more than a couple of bhp. It is even possible to lose performance as various magazine tests have shown! They generally have more impact when combined with a full exhaust, chip change and proper rolling road setup.
Cheers,
jv