Off topic: a guaranteed way to start a flame war in an automotive forum is to state that there is no such thing as "backpressure" and that fluid dynamics and the continuity equation fully explain why restricting exhaust pipe diameter results in increased performance.
It gets even more fun when the back pressure is in the intake side (aka boost). I was tuning a friends turbo 300zx after he upgraded to a bigger turbo. At the same boost levels as before the car was slower. He was confused. After a quick chat around turbos, air speed, etc., I simply told him we needed to crank up the boost. Some tweaks to timing and fuel maps and this thing could now really fly.
Light the fire and then sit back and enjoy.