I believe even if the radiator is fully closed it isn't actually closed - only that the flap is fully retracted for minimum drag. With the radiator 'closed' and at high throttle/RPM settings you can barrel along quite happily provided you keep the speed (and the airflow through the radiator) high. At lower speeds closing the radiator will cause all sorts of trouble.
I do a lot of P-51 1v1 practice and the engine will usually withstand 15-20 minutes of rolling scissors at full power before conking out, but it's incredibly easy to go vertical, have the speed drop to zero, forget to reduce throttle, and then *bang* the engine's popped due to no air going through the radiator.