I'm not sure of the exact reason, but heli guys who fly brushless motors have found that limiting RPM by running less than around 80% throttle or so at the transmitter tends to get the speed control into thermal issues.
I believe it's something to do with the effiency going down at partial throttle settings, but I'm not certain of the exact reason. I do know that heli flyers tend to spool up to RPM and maintain it for the whole flight, while we're on and off the throttle constantly, so maybe it wouldn't even be an issue for us... I'd ask someone at Castle to be sure.