Check your option list on your window sticker. See what it says about "axle". Usually it says what the ratio is, even if you have the base axle. Also, what trans do you have?
The Corsas are known for not droning. They are about the only ones that you can pretty much guarantee won't drone. Some others may not. The Touring is not worth bothering with as it is almost like stock. The Z06 only fits Z06 cars, I believe, as they have considerably different exhaust systems on C6s. Gotta love those cutouts! So that leaves the Sport. I guess they don't call it the Pace Car or Indy because it hasn't been used on an Indy Pace Car yet, whereas the C5 versions were. It's probably the same thing, the "loud" exhaust. It should suit you just fine. It should be nearly as quiet as stock when cruising, but quite a bit louder at WOT.
At the end of the day, though, only you can decide about a catback, and probably only after you live with it for a while. There are plenty of guys that have gone through 3 or 4 of them. Thankfully, they are easy to sell on the used market for most of what you paid.
For gearing, what I would do is buy an entire replacement differential from Dynotech Engineering. I mentioned before keeping your stocker, but that was before I saw the C6 core charge. Ouch! Anyway, check
this page. I see he has listings for 2005, and 2006-Mr. That leads me to believe there is something "funny" about the C6A6 diff. If so, and if that's what you have, you might be out of luck.