This should be promoted to "sticky thread" status !
Sorry I can't give you any empirical data yet, ( my router is STILL in the planning phase ) but this thread couldn't be more timely.
My gecko G201 drives arrived today. The instructions stated that the Gecko drive's temperature must not exceed 70 deg C (158 deg F).
A non-contact laser thermometer to monitor both the drive temperature AND the stepper motor temperature sounds like a wise investment.
I'm curious, too, if any CNCZoners use active cooling (fans, heatsinks, etc.) on their steppers.