Rudy's sells an 'in house' brand 5" downpipe back made out of 409 SS with an emissions-snooper friendly appearing cat device and all hardware for $349 shipped. They sell a 4" same thing made by a brand name I forgot for $400.
Your turbo outlet and downpipe are only 4" so other than sound it's not going to make you go faster but a 5" will definitely resonate better.
For another $80 they'll probably also sell you a really Mopard looking 8" tip for it in typical Rudy's overpriced fashion but for a git r done complete exhaust they got it.