This may be a stupid question but will a full catback make more power than just axleback? I didn't know if the stock resonators on the WRX are very restrictive? I know if I do the full catback i'll be better off if I wanna put a new downpipe/uppipe on my car... SO I dunno for now?
Pretty much wanna do an aftermarket downpipe first. The downpipe is where you make all your power, the catback just eases the bottleneck once you have one and brings out that nice boxer rumble. My suggestion? Get an aftermarket downpipe, then get a catback.