I know that a bunch of you are stage 2 already, but due to a large number of people who aren't asking us for catback exhaust recommendations frequently, I wanted to post up some details on one of the more popular choices available currently.
Invidia USA makes some great quality stuff for our cars including uppipes, downpipes and many configurations of catback exhaust systems. The "Q300" catback system is fantastic for the 02-07 WRX and 04-07 STI. This system sounds deep and "Boxer-ey" without being overly loud or obnoxious, most people find it to be the perfect balance of aggressive tone and reasonable volume. From tip to tail the Q300 is full SUS304 stainless steel using 76mm (3 inch) piping and a 101mm muffler exit. The muffler uses a straight through perforated core with a generous amount of packing and a "Flat Bottom" design that Invidia says helps reduce aerodynamic drag.
The price is really good for a system of this caliber and Invidia has a responsive US based facility with staff that are really easy to deal with. If any of you are looking for a good exhaust or turbo back system I highly recommend this be one of the options that you consider.
As always, feel free to let me know on here or email our customer care department if you have any questions!
I have this Invidia Q300 quad blue tip. The sound is amazing. I also have the Invidia high flo cat down pipe as well. I've heard other exhaust systems, and the thing I like about mine is that it is not obnoxiously loud but has a strong deep tone. It definitely is my favorite sounding exhaust.
"Turbo Back" systems can be bought as the 2 main components: Downpipe and Catback sections.
We carry literally dozens of different downpipes and Catback systems for the MY04 WRX so you can pick and choose the combo you like the best, but if you were looking to keep everything the same brand Invidia makes a few really nice downpipes for your car.