Correct back pressure does not result in lower power. Tuned pipes create much more backpressure than open face, no muffler at all, but there is a huge increase in power. It is all about the timing of the pressure wave that is sent back to the exhaust port to keep the fresh fuel/air charge from escaping while the exhaust port is still open. If a Pitts type muffler is not even creating enough back pressure for tank pressure that is not a good situation.