I see they make the after market entire cat-back exhaust system for the 3.6L, GXP sedan, but has anyone just went and replaced the stock muffler with like a magnaflow and left everything else? Just wondering if you can still get the sweet sound instead of a quiet one that comes factory.

yea there are a few different ways that i have seen people setup their exhaust.

there is the GT-R exhaust from Classic Performance

then there is the dumb looking one on ebay

then there is whatever you create...

Personally i am going to put on a custom magnaflow kit

here is a pic of that

http://s153586717.onlinehome.us/media/G6_Pics/G6_Ex_GTR1.jpg

Im on my third exhaust. Dont cheap out or it will cost you more in the long run.
Take it from me.