What do you think the danger zone temperature is for those mosfets?
The datasheet rating is 150 C = 302 f . Doubtful that you will reach that temp before the FET or something else fails. 2/3 of the junction temp is a good threshold so 200 f might be a good target.
Since you have the case open and fans laying around, I'd suggest to position a fan blowing down across the circuit board and measure some temps with your IR gun as you raise the voltage with the pot. Stop and leave an interval of time so that everything reaches stasis at a few points.
