It honestly doesn't even make much sense to assign a "power" value to something like an aircraft's propeller that produces a linear thrust force.
What's the propeller's power when airspeed is zero?
But you forget that top speed was reached by that historical horsepower, so, if it requires a much higher one to get same speed, something in flight model must be fixed, and that should not be power output, since also affecting climbing performance.
An aircraft doesn't really care what the shaft power output of the engine is. An aircraft's motion is solely determined by the forces acting upon it - lift, weight, thrust, and drag. Engine power
doesn't actually affect stable climb performance either - that, too, is simply dependent on the equilibrium of those four forces.
If we have a plane with a bad propeller that needs 150% of historical engine power to produce 100% historical thrust, and a plane with a historically correct propeller that only needs 100% engine power to produce 100% historical thrust,
both planes will fly the same. The absolute engine shaft power shouldn't even enter the flight state calculations. The plane that has "higher" power just has an engine that's doing 50% more of nothing. If the thrust is the same in both planes they fly the same (although the "high power" version may end up having higher fuel consumption, depending on how the engine modeling does its thing). If the engines are turned off, they both fly the same.
Anyway, if your working hypothesis is that the 150% engine power is used to offset 150% drag, there's a fairly simple way to test it - simply turn the engine off and see if the aircraft has excessive drag. This would be apparent as an abysmally bad glide performance. A good comparison point would be a similar aircraft not affected by odd power anomalies - a P-40M could be ideal, due to very similar airframe. If the two aircraft have about the same glide performance, then that would suggest that they don't have substantially different drag.
Since I don't recall anyone complaining that the early P-40 models have the approximate glide performance of the space shuttle, I very much suspect that the reason for the high power requirement is mostly not related to lift, drag or weight, but instead contained in the thrust component of the aircraft - and that practically means the propeller.