any engine with a turbo wil lperform best with the largest possible, or no exhaust. the turbo itself provides tons of backrpessure; after the turbo, you want the largest size piping you can fit/afford based on your HP goals.
__________________
kawasaki ZX12-R
'99 Dodge 2500 CTD 5 speed, 4x4, Greddy EGT, Edge EZ, BHAF, 4" exhaust, AirDog, running 80-100% biodiesel.