Downpipe and high flow cat wont change the sound, just give you more of the headache with a check engine light from the crappy cat converter.

The 3800 has a terrible exhaust note and its very hard to make them even sound decent.

Stay away from chambered mufflers as they just drone like crazy, I think some guys run a dynomax muffler and they are pretty tame, but I can't be for sure. Do some research.