I'm gonna go with everyone else and say Ganzflow
http://www.imprezaRS.com
For the performance gains it's better then just getting a K&N panel filter.
Your next best bet would be a cat-back exhaust, I did a custom Arospeed stainless / aluminized set-up for $201.16
Pulleys always get mixed reviews, the undersized ones don't seem to work as well and stock sized lightweight pulleys seem to also have flaws.
They're expensive, but a new ECU or a chip gives you potential to do alot more with your car and brings out more HP from your stock engine.
Or just go crazy and do a ZEX nitrous kit and shoot off a 40 or 50 shot.
Who knows, it all depends on how much your "buck" is.
Big Bry.