Dick,
Dave here.

In following the thread and in applying my full scale experiences here is my two-bits worth:

Horsepower goes down with altitude increases.
Horsepower goes down with barometric pressure decreases.
Horsepower goes down with lower humidty.
Horsepower goes down with temperature increases.

IE. Cold, Wet, low altitudes produce maximum HP.

Airplanes (non-turbo) fly FASTER at elevation due to cooler air and less air friction. They also turn the fixed pitch prop about he same RPM or perhaps a bit slower.

Airliners can not do Mach .78 at 2000 feet elevation, just too much friction on the fuselage. Drag increases non-linearly with increases in velocity.

Bottom line: RPMS are only one indicator of pwer. The better one is thrust delivered (read effective HP).

Have a great day.