you dont need an insanely big dp unless your runnin 500-600+ hp but the upgrade from a 3.5 inch system to a 4 inch system IS alot better trust me egts will drop even more, sound will be even deeper and better. mainly cus the bump in diameter, and mainly bc of all the sharp bends in the factory exhaust. just look at a pic of a 4'' aftermarket system on the ground next to a removed factory system. remember im talking about the system not the dp. 3-3.5'' dp is more than enough for most of us