The F.I. TDX and the Stillen are the two top exhausts as far as HP gains go, based on the dyno testing that the companies and users here have conducted so far.
As far as volume of the F.I. exhaust, you can change that based on whether or not you use an 18" or 12" resonated setup, or you can get non-resonated for the loudest option. The non-resonated one sounds like it is about as loud as it gets for exhaust options without going to the GT-HAUS for ridiculous volume.
If you haven't checked
here yet, give it a look.
My Stillen exhaust sounds great and the gains were good, however I wanted something a bit louder and deeper sounding... which is why there is a non-res F.I. TDX sitting in my GF's garage to replace it
