Power is squared as speed doubles. I've corrected the horsepower figures in my original response. Too much late night calculation effort involved. They should have been:

750 rpm - 1 horsepower
1500 rpm - 4 horsepower
3000 rpm - 16 horsepower

Now does an automobile engine fan ever really pull 16 horsepower? Probably not because the viscous drive clutches never fully "lockup". There is always some slippage which limits the top rpm of the fan blade itself. If it's a pure mechanical fan, bolted straight to the drive pulley, then the blades will flex, reducing the horsepower needed to spin the higher rpms.

You're correct on the stationary dyno situation emphasizing the fan load, but going to the electric fans was a major fuel mileage contributor when we brought out the "X" cars (Chevy Citation, Pontiac Phoenix) in 1980 at GM.

2002 X5 3.0 281,400 miles
2004 325i 120,700 miles