Kelly brings up a very good point, in regards to street driven vehicles. On a street motor the mufflers can dominate the engine's performance more than people realize. They can shift the powerband lower in the rev range, kill over-rev, dictate the need for a split pattern cam, limit the amount of overlap that can be tolerated and limit the horsepower output. The fact that restrictive mufflers shift the powerband to a lower rev range is why some people say exhaust back pressure improves low rpm torque and drivability.
I have no dyno numbers to back this up, but I would suspect that the performance benefit of a 180 degree exhaust system is largely negated when the system exhausts into mufflers. They still look cool however!
Have you ever heard the formula 1 whine an 8 into 1 system makes?
-G
