Whoa--Zombie thread. Meh, it's Friday night, so I'll give it a quick reply.
Since I actually run a brushed silvercan 540 in F1, and was running a 27T single in FWD, I would say that after toasting two motors so far this year, I would keep temps under 200 deg. F.
At around 200 deg. F, a few things I noticed have happened depending on how robust your motor is: The brushes get that purple hue, they are pretty much toast-replace them. When the commutator gets that purple color, it's 50/50 whether skimming a bit off the surface will ever bring the motor back. When the motor shaft turns purple, you are too hot and you have pretty much toasted the motor, including possibly damaging the magnets..