One of your option codes will tell which rear you have. It will start with a G. I expect you have the base diff, but I don't know what that is on a C6. On a C5 A5 it's a 2.73, which is pretty lazy.
A quick review of the C6 forum on CF reveals that I am out of my depth regarding C6 diffs...and apparently lots of other folks are too.
Much disagreement and misinformation. Call Phil at Dynotech, and ask him what you have and what your options are. They *know*, because they work on the things every day.
The Corsa is a catback. That is, it's just the rear portion of the exhaust, and does not include the "x-pipe" (a misnomer). That's fine, changing an x-pipe is not cost-effective. If you get longtube headers it will come with a new x-pipe, but other than that leave it alone.