i have a 3.5SE (with CVT) and i got 2.5" cat-back done to it a while ago. i kept the stock muffler and just did 2.5" cat-back to get the kink out of the pipe. some people were asking me how it sounds before so i finally recorded a video. nothing amazing (stock muffler) but a little better then 100% stock.
oh ya and in case you're wondering, i have under 5,000 miles on the car.... some where between 4,500 and 5,000. seems like it got a little louder after the last couple thousand miles
http://youtube.com/watch?v=i8VlOpjZ-wg