With propellers in the normal range, you can run your O.S. Max .91 engine from the high 9,000's to just about 11,000 RPM. Depends upon the propeller used. If you have a propeller that will let the engine run in that RPM range, and it lets you fly the model the way you want it to fly, you have the "correct" propeller.

15,000 RPM is much, much to high an RPM figure for normal running. That's where O.S. either found the power peak, or the maximum RPM you could run the engine before instant damage (the "red line" speed).