Let's just say I've seen a few dyno graphs over the years.
The headwork's not necessary but in a built motor with high CR and cams and such it's good for about 10 hp. Why not, eh?
Stock ITR's gain power from running 2.5" exhaust, a built motor making more power should go with it too.
I like to keep cars street legal, hence the cat. If you don't care about emissions testing run a straight pipe, but a good hi-flow cat isn't much (if any) of a restriction.