Why do you say motors loose efficiency above 40k RPM? Which motors exhibit this behaviour?

Motors run most efficiently when iron losses and copper losses are equal. Copper losses increase as torque load increases (current goes up), while iron losses increase when RPM increases.

Obtaining maximum motor efficiency is a balancing act of seeing how much torque you need, and running your motor so that its RPM is high enough to deliver the required torque after gearing and low enough to achieve an equal balance between iron and copper losses at these torque levels.

In e-heli world, high performance setups are chosen so that motors run just below their peak RPM where heat buildup increases exponentially due to iron losses. At that point the motor delivers the highest torque after gearing before copper losses become too high.