I have seen two different formulas that predict the engine HP using pitch, diameter, rpm, and a divisor. Checked them both out against the power/torque curves in published engine reviews, and found that both are within 2 or 3% of the measured HP around the middle of the RPM band that was measured, while one drifted to almost 10% low at the low end, and almost 18% low at the high end, and the other one was just the opposite.
There is a formula to predict the maximum speed of a plane from prop pitch and rpm, but whether or not a given plane will even come close depends on a lot of other factors, including weight and total drag.
In general, the engine that turns a given prop at the highest rpm is developing the highest HP. That doesn't mean the highest power engine is best for a given plane. A high performance .91 might turn a 9-7 prop at 25000 rpm, but you wouldn't replace a milder .40 turning that prop at maybe 15000 if the plane were a 4 pound 500 sq" Quickie 500. The 91 would be too large and heavy to be practical.
As far as unloading of the prop in flight, one of the previous reviewers for MAN was set up to instrument his engines and monitor them through telemetry. He said people seem to get upset with him when he reports that very few engines unload more than maybe 500 rpm in flight, and even most tuned pipes don't increase RPM as much as people like to think.