There are no prop restrictions in the rules. The problem is that to get the same theoretical top speed with only 2/3 the rpm, you would need 3/2 or 1 1/2 times the pitch. Not a problem once you get up to speed, but before takeoff when forward speed is zero, the prop is stalled because the angle of attack (pitch) is so high. This gives very poor acceleration. This is why real airplanes have props that can change their pitch. For takeoff and climb they use lower pitch, and for cruise they use higher pitch. With a high revving two stroke, you can use less pitch for good acceleration, but due to the high rpm, the pitch x rpm still gives a high theoretical top speed.
(Sorry to all the engineering types for the gross simplification.)