On my sti (turbo back taken off my old wrx) i'm currently running an Invidia downpipe with a Megan racing catback going to be protuned w/ the Accessport. +1 for Invidia and Megan racing. It's a really tame note even at wot!
It's a pretty easy job... drill a hole, weld in a threaded piece and plug it with a threaded plug. I don't know the going rate for a few minutes of labor, but you could probably have that done for 20 bucks (+/-) if you shop around.