Here's what Kevin Cameron has said about restricting the outlet. "Exhaust pipes always create backpressure, but the more there is, the higher the amount of hot gases that won't be able to exit the cylinder while the exhaust port is open. Its heat, added to the fresh air/fuel mixture that enters the cylinder, may be enough to put the engine into detonation.
Sometimes a 1 or 2 mm reduction in tailpipe ID will result in a couple of extra HP,but the extra heat may make the engine detonate after a few seconds."
Sometimes a 1 or 2 mm reduction in tailpipe ID will result in a couple of extra HP,