I had a 2.5" downpipe with 3" Magnaflow catback and it sounded really good. I dont have a sound clip laying around, but i assure you it was deep and burly. I had 2.5" exhaust for a while and i noticed a pretty good difference when i went to the 3" back. Seemed a lot snappier and the turbo spooled quicker too. With my new setup Im going to run a 3" downpipe and then throw some scraps together from some diesel kits we do at my shop and go 4" back. Is it necessary, no. Why would i do that?? Cause i can.