Backpressure is bad, but it is a direct result of a high speed flow exhaust. People often muddle up their 'backpressures' and 'gas momentums'.
It's a bit like saying "big tyres give more grip, but also more drag, therefore drag must be a good thing". It isn't, but we just accept that. These two quantities come together (e.g. RC40 has more backpressure, AND more flow).
One of Newton's Laws (First of thermodynamics?) says that every action has an equal and opposite reaction, therefore if your exhaust gas has momentum (INERTIA sucking gas out), then it also has INERTIA as seen by the piston trying to accelerate it. This is seen as backpressure.
You will never get MOMENTUM, but not BACKPRESSURE. Backpressure is the REACTION of the momentum, or NEGATIVE GAS INERTIA.