This is from the OS website.

Assuming the engine is fully broken in, you are using good quality fuel with at least 18% oil, and that there is no fuel foaming occurring, it is possible you are overpropping the engine. With use here in the U.S. at our altitudes, fuels, etc, we have found that this engine prefers slightly smaller props - 13x8 or 13x9 specifically - and turning at higher RPMs. If your engine's max RPM is under 10,500 then the engine is working too hard and not making peak performance.

I read it this way... Correct me if you see something different...

Use a 13x6 to 13x9 and shoot for at least 10,500 rpm. Are they saying that perhaps anything beyond a 13x9 would likely not break 10,500 rpm? If not, what are they saying here?