If I mess up the EFI System Partition, will Mac OS X be unbootable? I forgot to make a backup of my EFI System Partition (ESP) and  have installed grub onto it.

If I happen to accidentally mess up the partition, will OS X be bootable? If not, how would I restore the EFI System Partition without having a backup?
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.