Lets use a standard prop from a Cessna 172 as a baseline for this.
If the prop turns at more than X RPM (around 2500 I think) the tips go supersonic and that is very bad for the aircraft. If a 50 HP engine can turn the prop at its max speed, what good does more horse power do? The prop can not go any faster without causing damage to the plane. Its not going to be moving any more air because its size and or shape has not changed.
I know there MUST be a logical reasonable explaination for this but it escapes me right now.