you dont *have* to have the catback portion. I had a friend with an STI and COBB said it would be okay with just a downpipe, a little less power, but it will still run fine.

also you can get your accessport protuned for $$$$, but you aren't going to see huge power gains. If you have a wrx, which I suspect you do by wanting more power, you aren't even going to beat stock STIs on that turbo