What I'm doing is as follows:
1) set all "real" interrupts at priority 1,2 or 3.
2) use unused interrupts at priority 0 (lowest) to run the stuff that is more important than the main loop.

example:
when the serial receive accept a character it stores it in a ring buffer and set the timer 2 interrupt flag (T2 is not running). T2 ISR at the lowest priority process the ring buffer and when the message is fully processed hands it off to main. This way I keep the ring buffer process active more than doing it in main and still without affecting the real important interrupts lag.

Erik