I talked to a lot of people including maldon about this but ,nobody has any solution if you wanna upgrade your gpu go with RX VEGA 56 or 64....
Rx 560-70-580, have the same problem without IGPU which we don t have in our Xeons.
They are supported on OS level as external gpus FOR MACBOOKS WITH THUNDERBOLT AND ALSO FOR OLD MAC PROS.
The problem comes when you attach them to any real Smbios (like ImacPro or MacPro6.1 etc etc) it freezes your mac on some sites because of the hardware encoding and HEVC support missing.
Trust me I researched this for a long time the freezes were driving me crazy.
I have unlocked Xeon E5-2696 v3 that perfectly works on 3.5Ghz on all core under load - Cinebench test shows 2900 on Win10. Under Os X 10.13.6 it gives me only 2.9Ghz on all cores and 2600 score. CPU microcode uploading during POST - x39 version, mac version of microcode is x3D. Could it be that, mac microcide is overriding x39 and limiting CPU max turbo boost frequency? Is there any way to remove it from mac?