Not much IMO. The newage cars have a cat in the upipe and Ive know alot of them to get an uppipe from a classic and use that just before mapping which does the job. Sorry just the newage WRX has a CAT and the newage STI does not.
Also if you get an uppipe to fit with your standard manifold then if you decide to get an aftermarket manifold it comes with a new uppipe. The one that fits to standard one normally wont fit new one.
Damo