Windows 8.1 EFI on 2012 13" MBP
Two late 2013 rMBPs (15" and 13") with EFI Boot and 5.1.5621 work flawlessly with this and can switch between OSX and Windows via Startup Disk and Boot Camp Control panel.
The same implementation on a 13" MBP 2012 works, with some issues.
1. No sound.
2. External display via TB using DisplayPort will not see external monitor (Intel HD 400 GPU).
3. Startup Disk on OSX gives 'No bootable device' error, but Control Panel -Boot Camp on Windows side will switch to OSX without any issues.
4. Trackpad has minor issues.
All machines have the MSR partition. On the 2012 MBP, with the ALT key, the Windows partition shows up as EFI Boot and will boot Windows 8.1.
I have also tried the 2012 with Yosemite beta, but it makes no difference.