FWIW,many people forget the motor is a system. Improving only one part of a systems can be detrimental to the system as a whole.
For example, on a motorcycle, a better header (less back pressure) will deteriorate performance if you don't rejet the carburetor to make the mixture richer. With a rejetted carb and headers, performance is better than original. A lower back pressure exhaust changes the nature of the mixture in the cylinder. If no compensation is made in the rest of the system, you may see a deterioration instead of improvement.
Look at fuelie dragsters, they use short pipes, one per cyclinder for the lowest retriction possible. If restrictions helped, I am sure they would have some.
