If an engine is propped below its RPM of maximum torque, it is going to unload a whole lot at high speed.

That prop is the size when going one size up in the props, you lose a lot of RPM.

For example:
You change from 11x4 to 11x5; you lost 500 RPM.
You change from 11x5 to 11x6; you lost 400 RPM.
You change from 11x6 to 11x7; you lost 500 RPM.
You change from 11x7 to 11x8; you lost 1,400 RPM.

I used increasing pitch numbers, but increasing diameters will act about the same, with 0.5" changes and the same pitch.

Lets just say you use the 11x8 prop. As speed is gained, not only is the load reduced, but also the torque is increased. So the gain in RPM will be larger than if you used, say, an 11x6.

BTW, as much as I admit Downunder is right, you still have to open the needle to allow for even more fuel, the consequence is a lean run.

Sincerely,