The shift kit would definately help, used to have a 25GT and would have liked crisper shifts in it.
I'd say for cams, porting, e-manage + tune you'd be looking at $3500 at the very most, some good shopping around will drop that price for sure. While you've got the head off the car, would maybe be worth having it decked and put the compression up a notch. You'll only be able to use 98+ octane though.
An exhaust can vary greatly, my CES exhaust was $2K, but it is awesome
I think you may be competative with stock GTts for sure, you've got about 100kg weight advantage over them.