I have flown aircraft where the engine power was measured and set by: Engine/Prop RPM, (fixed pitch piston props),
"Inches of mercury" Manifold Pressure (constant speed piston props),
Percent engine RPM (simple jets),
Torque pressure in Foot-Pounds. (turbo-props)
Torque pressure in PSI, (turbo-props)
It doesn't really seem to matter what the units are, or even what they measure. Ultimately they are just the numbers you expect to see or expect to set, under various conditions of flight.
I don't really see how I would be better off with the instrument indicating horsepower, (or kilowatts), or what I've currently got, which is psi. It is just a number, like they all are.