On wheels, looks like you can push offset up to + 7mm (.275")
Never noticed the part before about diameter ok to deviate by 1"! (13.4.B)
http://cdn.growassets.net/user_files/scca/downloads/000/019/008/2017-01-17-section-13-street-category.pdf?1484692275
What's the "hot" tire to have nowadays, anyway? Back when I was into it, there was no minimum 200 treadwear requirement, but they also were calling "street" class "stock."
You could potentially take the widest 18" wheel set offered in 2001 for the Boxster, and make it into 17" +.375" offset, if that would be a good tire.
13.3 says "Tires may be replaced with any size that fit the allowable wheels"... for gearing change, and lowering of center of gravity, you could potentially consider smaller circumference, if available in a "hot" tire compound.
But I don't know if that might make you need to shift into 3rd gear too much. Curious how you end up setting the car. Don't neglect good (budget a good bit of $, imo) shocks, and some
fresh bump stops.