UEFI-capable custom firmware is now available for most ChromeOS devices -- see: https://mrchromebox.tech/#devices.
GalliumOS 2.1 fully supports UEFI booting on ChromeOS hardware running MrChromebox's UEFI firmware.
The GalliumOS 2.1 ISO for Sandy/Ivy Bridge boots to the GRUB shell, but this can be easily worked around. See https://github.com/GalliumOS/galliumos-distro/issues/342.
Earlier releases of GalliumOS did not include UEFI support.
UEFI booting reduces boot time by several seconds (compared to Legacy/BIOS mode).
UEFI might also be a reusable solution for future Braswell support, and possibly for some ARM Chromebooks and non-ChromeOS devices
NOTE: UEFI support is not included in factory firmware on any ChromeOS devices. Adding support will require flashing "full" firmware, and will therefore prevent dual-booting alongside ChromeOS.