You have to tune for them because our cars have an maf. (Mass air flow sensor). Wrxs are very dependent on the ecu for real time adjustments to boost, timing, etc... So if you put one on there and don't tune for it the ecu will not know how to adjust due to the turbulence and extra air that come in. Honestly the stock air box and intake is fine for lots of power...well over 300 hp. If you just must put one on i would go with the Cobb short ram and the extra air box you can get with it. This one won't mess with anything like a/f ratios or cause turbulence at the maf. As far as the exhaust if you want quiet you can always just get a downpipe and get an adapter flange and run the factory cat back. Those sound nice and mild. Good luck man.
Cobb ap stage 2 ots, catless uppipe, catless turboback, Cobb Sf intake.