I've accidentally gotten motors that hot, and as long as you get it cooled down quickly, there is not much danger. Richen your high needle, or cut a bigger hole in the body. Also remember there is discrepancies between temp guns and loops. Hell, a friend of mine has the same gun I do, and his reads 3 degrees higher than mine. With all my motors, I've tried to keep them below 280, just as a rule. You can judge by sound too- if it sounds as if the motor is "cooking" or "sizzling", cool it down quickly, or shut it down immediately. Those sounds generally indicate that you've broken the 300 degree mark.
Hope that helps.

Matt