I found this:
https://www.insanelymac.com/forum/topic/329390-amd-firepro-w5100-on-macpro-51-high-sierra/
Seems that the card will work with these mods, you will just have a blank boot screen until the system starts.
I'm planning to change my FirePro W5100 with a Quadro M1200 because I need CUDA for what I'm working on.