[lmm]

> It must be less efficient than on earth

No it mustn't. You'd expect efficiency to be higher, all else being equal, since there's less drag to overcome - aircraft can generally fly more efficiently at high altitude on Earth (up to a point). The limiting factor is that helicopter flight only works while the rotor tips are subsonic, so as the air gets thinner the "IAS" that a pitot tube on the rotor (that's going as fast as you safely can) would experience gets lower and lower, and so the flight dynamics will be like a helicopter with a slow rotor, but speeded up. But for a small helicopter the square-cube law is on your side and so a low "IAS" is quite doable.