Interesting point. Of course a wind-powered device cannot extract all of the power of the wind; the power goes more like v^2 times (delta)v, where the delta is basically how effectively the device can slow the wind. Of course, one might expect this term might be proportional to the original windspeed, so it still goes roughly as v^3. For a related analysis, see Betz' Law: https://en.wikipedia.org/wiki/Betz_law