suggestion to Intel team:
it seems to me that Intel boards Z77 and Z87 are all sensitive to graphics cards that do not have a UEFI GOP capable vbios. There seem to be many posts with people disabling legacy and then having a lockup of some sort. I know my Z77BH-55K had these issues as well. I even had to send back the board once. With the DZ87KLT-75K i had severe boot and graphics issues as well when enabling UEFI mode which became much better when I got a GOP bios for my videocard.
Seems to me that it's a good thing to mention in the manual that GOP is a requirement when UEFI is enabled. Other manufacturers have a CSM module which can be turned off as with these boards but if the board detects a non GOP bios it reboots with the CSM re-enabled in someway.
Let people know GOP is essential, maybe even build that into the bios. If a user enables UEFI and disables legacy, test if the PEG has GOP capabilities and generate a warning.