So, was delayed a bit, as I realised after the card arrived that I had thrown out my PSU modular cables! So I had to order one in.
Radeon PRO WX 7100 installed in second PCI-E slot after I first removed Nvidia Drivers from macOS High Sierra 10.13.6. Plugged it into my display using second DisplayPort - so both cards are attached to the same 4K display using two DisplayPort cables.
Booted but appeared to hang with the Apple Logo and the progress bar incomplete. Enabled verbose in clover args and it stopped at:
"ConsoleUsers: gIOScreenLockState 3, hs 0, bs 0, now 0, sm 0x0"
This is I don't think much of a concern... out of interest, this was using the DisplayPort from the GTX 1080 in PCI-E 1. I used the display OSD to switch to the other input - and voila - I see my High Sierra background.. but no login screen. I have a go at logging in using keystrokes, and as far as I can tell - it works, my HDD whirs.... but the UI doesn't move on in any useful manner.. hmm.
Next step, I wanted to pass nv_disable=1 via Clover Boot Args. However for some reason I couldn't type _ in clover. There are a few threads about this, but none helpful. So I booted using macOS recovery partition and was able to mount my EFI partition using terminal, then vim the config.plist to add my nv_disable=1 to the boot args.
After this, it boots fine - leaving my main display at the same stage as above (ConsoleUsers: gIO.... etc) - HOWEVER when I switch inputs, I find the macOS login screen with the accounts displayed etc. Login - all good!
So my next steps I think will be to tell my bios to start initial display on the Radeon rather than the 1080 - I am hopeful that maybe that'll let it start up by default on the right input. {EDIT: Tried this, actually makes my machine not boot at all! } I guess I'll look at a DDST edit to disable the GTX for macOS... though I'm not clear if a DDST edit affects only macOS or whether it would also flow through to Windows?
If all else fails on the final niggle, I'll live with switching inputs when I boot to macOS. Catalina install is near in my future....