If the timer was set to the highest priority the function can not be interrupted by an other interrupt if you don't allow it (is default).

I think you want to know to be shure that the interrupt is always handled in the 0.48ms you mentioned.

You should also check interrupt latency (not only execution time). If you have two or more interrupts at the highest priority an other interrupt may be active when your timer interrupt occurs. Your timer interrupt must then wait for the other interrupt to finish. So your interrupt will be handled later than you expect.

Happy programming.