If I mess up the EFI System Partition, will Mac OS X be unbootable?
If it happens you mess it up, you can download it from Apple.

EFI and SMC firmware updates for Intel-based Macs

This article lists firmware updates that were released for Intel-based Macs. They update the firmware that originally shipped from the factory. If your computer isn't on this list, then you don't need an update to the factory firmware.