Incidentally, it is useful to avoid confusing backpressure with scavenging. Scavenging is good and backpressure is bad, but the two are often confused.

Yes, I know, but using the term backpressure is the simplest way of communicating the idea, as that is what everyone hears you benefit from by reducing it by going with a better flowing larger exhaust.  The industry really needs to stop talking about reducing exhaust backpressure, and use increasing the velocity of exhaust gas flow.  The best system would be an active exhaust system that automatically increases decreases exhaust velocity due the the throttle input, something like a butterfly valve linked to the throttle position sensor.