[Dafni]

Of course. But according to Mike, BL motors are most efficient around 35'000rpm.
Sweet spot and Most efficint spot are not necessarily the same. In fact they are two different things. YOu can "tune" the sweet spot by gearing, but max rpm is given by voltage. Just make sure the sweet spot is in the efficient range :)
The best is to start somewhere and observe the temps.

Let's put this sweet/efficint thing another way: The sweetspot concerns the gearing. It's an equation on gearing, weight etc.
The efficiency is a constant at a given voltage, if I understand it right.