You can calculate the heat losses at different voltages and see how the motor will behave.
There are two basic losses that MotoCalc and most people use that are fairly simple and more or less accurate. They work very well in the middle range of rpm and power. At hi power loadings (near current saturation) and hi rpms they are on the low side.
V = voltage applied
Io = no load current (published Io is usually measured at 6 - 9 volts - so you need to check it at your actual used voltage. It can varry a lot sometimes)
Rm - motor resistance
I = opperating current
All loses are in watts
Iron losses are = V X Io
Copper losses are = I^2 X Rm
In the case of the Hymax 2025 3200 at 300 watts on 6S (3.5 volts per cell assumed)
iron loss at = 21volts X 1amps (guess) = 21 watts
Copper loss = 14.2 X 14.2 X .144 = 29 watts
Total loses = 50 watts
Increase the current to 20 amps, drop the voltage to 5S and total heat generated by losses goes up to around 75 watts.
One factor these formulas DONT take into account is that most all brushless motors have an additional drops in efficiency that starts to come into play at very hi rpms and at near thier max power ratings. 5S and 6S are pushing them to the very upper limits and they will have even more heat than these numbers show.
Think how hot your 25watt soldering iron getts
Larry