Mac Pro 5.1 Windows 10 EFI Boot (Separate Drive)
I have installed it on 2 separate drives. Windows being in EFI mode, and during the installation I did not use any Boot Camp crap, but I had to leave only 1 empty drive for Windows 10 install and boot from Windows 10 DVD in EFI mode. Then I created the Windows partition from scratch and installed Windows.

Download the bootcamp drivers from this link. I did update Windows and installed Boot Camp 6 drivers by running msiexec /i BootCamp64.msi from Administrative CMD window, which is located in Boot Camp\Drivers\Apple directory. Then I connected all the other drives.

In macOS High Sierra I installed Boot Runner from Twocanoes. From the recovery partition's terminal I executed:

csrutil enable --without nvram

so the Boot runner can boot Windows. After logging back into macOS High Sierra, logout. You will come to the Boot runner's startup disk screen. Select Windows and it will reboot to Windows. If you want to go back to macOS, select macOS High Sierra option in the startup disk of the Boot Camp manager (in Windows)

I have migrated my internal Windows drive to Sonnet Tempo SSD Pro Plus SATA III PCI Express Card. Unfortunately, due to the fact the drives on this card are recognized as external, I guess Boot Runner does not populate an option to boot directly from this drive. As a workaround though I can select the SSD as a boot drive from the Boot Camp manager in my internal Windows drive which works like a charm.