After further investigation I am not entirely sure Fast Boot is the only issue here. Something is wrong with CSM/UEFI driver/OpROM support in both 2003 and 2015 that I tested. To elaborate:
After further testing it seems that even with Fast Boot off (which plainly refuses to work and keeps loading Windows forever) I had a constant appearance of Debug Code 64 upon REBOOT. Even if rebooting from saving UEFI settings. The only way to recover from it was discharing power entirely from the board. Upon first boot it would work but upon any subsequent boot it would get stuck at Debug Code 64. Troubleshooting steps included clearing the CMOS several times and reflashing 2015. Nothing worked. I downgraded via Flashback method to 2003 and noticed the exact same behaviour. Downgraded via flashback to 1404 and everything is working as intended.
After doing a little research and reading this document from Intel:
http://www.intel.com/content/www/us/en/architecture-and-technology/unified-extensible-firmware-inter...And this Microsoft white paper:
http://msdn.microsoft.com/en-us/windows/hardware/gg463092.aspxAnd considering that Debug Code 64 is CPU DXE initialization step I'm fairly certain that there is a problem somewhere in the CSM/UEFI driver/OpRom support as UEFI Driver loading happens during the DXE phase and CSM configures what is loaded/initialized (UEFI driver or Legacy OpROM). This looks like bigger than what I initially thought. With version 1404's option of just selecting EFI-Compatible option (can't recall the exact name right now) for the optional ROMs it works great.