My experience with the Orbit 20-10 motor is that it runs hot no matter what pinion I use. Have tried 19, 20 and 22 tooth pinions. I think I tried an 18 tooth as well. Heat is not noticably different. I have no integral cooling fan on the motor, just a hole in the canopy. Have stopped worrying about the heat and have been focusing on flying. Have not noticed any degradation in motor performance. Though, not scientifically tested, my impression has been that the motor runs cooler with the 20 tooth than the 19 tooth pinion. The temperature with the 22 tooth did not seem much different either. I have had motors get hot in the past if they were not loaded enough. Don't know if that could be going on here. I think I will try the 20th tooth pinion again as my flying skills improve as I liked the power and response. Don't know the actual headspead with the 19 tooth pinion as it has not been measured. With the 20 tooth, the head at full (or close to full) throttle during hover was measured at 2260 rpms. If one extrapolates: (19 divided by 20) mupltiplied by 2260 you'd probably have a good idea of the actual head speed. I find the 19 tooth very nice for my current flying skills.
Am wondering if going too high on the pinion will surpass the amp rating of your controller. Set it to insensitive and push it hard while you are low. Perhaps even on the ground with full throttle and full negative collective to see if the controller cuts out. I was torn as to which Castle Creation controller to use, the hv45 or a regular 80 amp controller. The latter is serving me well so far, but I'm at the max number of cells for that controller which is OK with me. My full collective is currently plus and minus 11.5 degrees using the 19 tooth pinion.
Do you have the integral fan on your motor? I've mostly been flying in 60 degree farenheit or less temperatures. It will be interesting to see how hot the motor gets in the heat of summer.