Joe is correct, You should be running 4 cells with a k/v of 960 at those rpm.
At the moment your ESC is running at 60.1% which is way too low. Everything will run hot.
If you use 4s, it will run at 75% which is right in the ball-park.
What Igor and Mark said. I can't resist tossing in my two bits, because I design motor drivers that are similar to ESCs -- but it all really boils down to is a highly detailed way of saying "more voltage = more energy lost to heat".
The average current (and voltage) at the motor has to be the same -- otherwise it wouldn't generate the same average torque. The ESC regulates voltage to the motor by chopping the battery voltage. With a higher voltage battery, the pulses that the ESC sends have to be narrower.
With these larger-voltage, narrower pulses, the current in the ESC goes to a higher peak value and dips to a lower minimum value. Because of the way that heat is generated in a resistance (I
2R losses is the key phrase), this means that more heat is generated in the motor (and energy is consumed to do it). Also -- and I haven't tested any motors for this so I don't have a good handle on it -- the higher applied voltage on the motor windings will cause more energy to be dissipated in the steel armature of the motor, both from eddy current losses and hysteresis losses.
Higher voltage on the ESC means that there's more switching loss in the ESC transistors. Whether this is important depends on the detail design of the ESC, so if I wanted to know for sure if this was an issue I'd be measuring temperatures on a real one. The same higher current peaks and lower troughs that happen in the motor happen in the ESC, which leads to conduction losses in the ESC (and the conduction losses are mostly the same kind of I
2R losses as in the motor).
What I have not done, because it takes a lot of picky, detailed, time-consuming work, is to try to quantify how much of this happens in an electric power setup, and how much of it comes from each possible cause. If I did do it, I'd just end up putting detailed numbers to what Igor is saying.
If batteries didn't fade toward the end of a flight, and if you didn't need some voltage overhead for power in the overhead maneuvers, then you'd want to set the thing up with as little excess voltage as you could -- but it's more important to have plenty of oomph in the clover and overhead 8 than it is to have the Most Efficient Setup.