Exhaust Backpressure the myth

Back pressure is not good for engine performance at any rpm! the piston try so hard to push the exhaust gases out, why would you want to put extra pressure on it? The speed of the gas flow and air wave are what it really needs.
A bigger and less restrictive tube might benefit higher rpm exhaust flow, but at low rpm it will allow the gases and the waves to expend, cool down and slow down too much, waves messed up, momentum lost, engine works harder to push the exhaust gases out. In this situation, the valve can restore some restriction, speeding up the gases and the waves. these waves carrying a lot of momentum actually helps to suck out the exhaust gas from the engine, improving power.