Your best bet, and I believe the direction we're trying to point you in, is to build your car toward a basic "Stage 2" setup (downpipe, tune, and an up pipe, in your case). You can use your stock cat back for this setup since it's not horribly restrictive or you could get just about any aftermarket cat back that you like the look and sound of. Usually, it's at this point when people begin to start looking at aftermarket headers, intakes, fuel pumps, injectors, bigger intercoolers, and other supporting mods for a bigger turbo. Then you'll be able to run 17+ pounds of boost safely and effectively. It sounds like you might start to be getting caught up in the numbers game... resist the urge, my friend. Tune properly and the numbers will come.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)