If you found a timing (for an fdr) that is the max heat you want that motor ever to run you know the amps/rpm (torque constant) or thermal time constant. Once you have enough slopes plotted for a series of fdrs you know the ability of your motor to dissipate heat. This can be a line through the rpm/torque slopes you created that limits tells you how far you can advance timing above peak efficiency for an fdr. It will work for 3 or 4 fdrs (depends on slope up to zero and down from zero) if those fdrs are within the bell curve for peak efficiency. If the thermal time constant or peak power curve is too complicated to understand before you try this then just get peak efficiency and FDR the car to the peak efficiency you calculated then go to slightly lower fdrs in increments until the car gets hotter.

the motor will get slightly hotter if you dont reach the rpm you calculated but it will be slow and potentially much hotter if you exceed the rpm you calculated. Amps/rpm usually dives very quickly at higher rpm on most motors but there are a few motors that the slope down (negative) is about the same as the slope up (positive) but that is pretty rare.