Yeah yelloeye has a good point but I've ran a custom catback since the first year I got my 06 LS m/t brand new. It finally rusted Infront of the muffler last year at the bugger weld after 10 years of Alaska winters so it wasn't so bad. A direct factory bolt on should last much longer so I went that route this time with a DC sports this time along with a new catalytic converter since the factory didn't have a 2 bolt flange only welded together.
I really liked the magnaflow sound(they've got 3 levels of mufflers I believe now, they only had one compact sport muffler when I got mine) but the DC sports sounds awesome and much cheaper when I found it online.