If Discopope doesn't have one, I have a GPmoto y-pipe for sale for $60. It's in very good condition and you won't have a problem fitting it onto your 04 WRX.
I let you know if I'm interested, I'd actully like to try and do all three that normally come in the kit, but really the y pipe alone I think will solve the problem.
^^alright. Xcelor8, the y-pipe is really all you need, but I have the bypass hose if you need it too (I just need to remove it from my car). Let me know.