I have quite a few upgrade questions for everyone. I am looking to get some more low end power out of my 04 wrx. I have been reading here for a long time, and had a 02 wrx that was totalled so I am not completely new to this, I just don't know much about tuning the wrx. I have been reading a lot about it, but still need help.

I want to improve on the breathing of the car with intakes and exhaust. What should I replace to get the best gains for my $? Up/Down pipes, cat back exhaust, cold air intake, K&N filter? I eventually want to replace all of this, but for now where do I get quick gains?

I also want to keep the car rather stock so I really don't want to mess with the ECU. What happens when I change something in the intake or exhaust, but leave the ecu alone? More power, but not the full potential I beleive, but I could be wrong.

Also I know there are 3 cats in the car. One in the exhaust, hence the cat back, and the other 2 in the up and down pipes. If I get rid of any cats I then have to have that little electric gizmo to kill any O2 CEL codes right? Also if I replace just an up, or down pipe, which to do first, and how do the effect of replacing one differ from the other?

Thanks for all the help.
Ben