It is very unlikely that the motors are too hot. Most stepper motors are rated for something like 100 deg C case temperatures (212 F).
If it is stalling during acceleration it probably has nothing to do with the max feed rate. I like the way you did your determination for max rate and accel. It is exactly how I do it and I used to use 80% of the maximum achieved as well, but have since decided that about 60% is more appropriate. So, you may want to slow them down a bit more, especially acceleration because on a reversal of direction there is a lot of "jerk", and Grbl doesn't implement a s-curve type of acceleration. So it is pretty easy to exceed the accel capability of a system due to the jerk created especially at direction reversals. See this for a brief explanation: S-Curve Acceleration and Deceleration - National Instruments
Which stepper drivers are you using?