What you say is true, but again its not backpressure.

Saying you need some backpressure for low end power is incorrect

Saying you need proper exhaust velocity for low end power is correct.

Yes, a pipe designed for low end power will be smaller, which will cause backpressure at a higher end.. but its not backpressure that gives the low end power. Thats what people have been saying this whole thread. Backpressure is the side effect to having an exhaust system designed for low end power (why you would want this is beyond me, but this is not what we are debating).

Its like saying you need tires that wear quickly for good cornering. Which is not true, you need sticky tires that are often softer rubber and the side effect is they wear out quickly. Its not the fact that the tires wear quickly that gives you good cornering, its the soft rubber.