I have a header, Magnaflow cat, magnaflow muffler & 2.25 piping from the cat-back. It is a little loud for my taste, but it purrs if you keep your foor out of it & that's about it. I don't think it frees up much power, just a few ponies, but the motor revs easier in the higher rpms in turn.
I think the most important thing about an exhaust is to get it to tuck up & clear rocks & stuff, and you might as well run a cat.
FYI The LC catback included enough pipe to delete the cat if you wanted to.