I got most of the tips from the article on SVTPerformance.com, you can read it here:

http://www.svtperformance.com/forums/how-230/568983-how-polish-your-exhaust.html
Before Photos:
After Photos:
I cheated a little bit as I only polished with paper up to 800 grit then used a buffing wheel to finish it.  My suggestion would be to
try some spots all the way to 2000, then a spot at 1500, and a spot at 1200 etc, and see which one you can live with.  The clamps
have about 4 hours just in them and the whole cat-back probably has 30 hours invested.