[RC-Monster Mike]

40k rpms isn't so much an efficiency limit, Brian. It is just that higher rpms yields higher amp draw and higher heat in almost all cases. This includes the speed controllers, which have to "think" at the motor rpm to keep time. 30k to 40k is a nice place to be - plenty of rpm to have smooth control and a wide range of gearing options to tune, along with decreased chance of overheating or motor damage(if you scream the motor at 65k rpms and jam the brakes with a heavy truck in a high traction situation, the speed controller will hate you, and you risk throwing the magnet on the motor - better to keep things on the safe side IMO).