Sorry to be a dick but there's just no power restriction back there. The only power restriction is the downpipe. The wastegate exhaust stream merges back in at like a 90 degree angle on the stock pipe and it costs you power. The cat back has been shown to be zero restriction with the stock turbo. All it is is quiet. Even if you want something a little louder, there are plenty of aftermarket exhausts with mufflers. If you don't want a muffler for other reasons, well, see my last post.
If you want performance gains, you just need a downpipe.
04 STi 16G powered lagless fun
04 WRX: SOLD