Hey Polaxgr, if you have 2 x D700 like me the best simple thing to do is :
1. Completely Disable "AMD Crossfire X" in the "AMD FirePro control center" on Windows/Bootcamp
2. Reboot
3. Game !
Windows and Games support for dual graphic cards like the D700 is extremely bad, so by disabling "AMD Crossfire X" you'll use only one D700 (6GB instead of 12GB).
But Games and Windows in general runs 10x better.
This way I can run Witcher 3 with Ultra Settings perfectly smooth.
It fixed also some graphic issues I had in Windows.
And the best news is : it runs way less hot and it's twice more silent, it reduces about half the fan noise I had before. Amazing.
Good Luck to you in the Mac Pro 6,1 Adventure ! Hope it helps !