GSSR is right, you want a good, straight through design when using a turbo. The less backpressure you have, the more efficient the turbo will run (unlike a normally aspirated engine which requires some backpressure for scavenging). If it was even half legal it would be best to run 2 1/2" pipe or better without any inline restrictions... but that's not exactly going to get you an inspection sticker anywhere in the US...

Will