Cat back exhausts will not effect any sensors to throw a code. They are all upstream.

The CAT backs do make the sound better/deeper, but keep in mind inline motors (4s, 5s and 6s) do not growl or thump, they buzz/whurr. My magnaFlow sytem does not sound like an import fart pipe.

If you do a PCM tune, Cat back, and intake on the I5, you will have pleanty of power, not like a V8, but much better performance than stock. Kinda closes some of the gap between the 8 and 5. Problem is when you do the same mods to the Alpha, youthen have much more performance with the V8.... those are the problems everybody likes.