Hi Middleman,
I have this same system running 11.4. I am trying to install a new reference AMD Radeon 6800 with no luck. Tried all the agdpmod=pikera, -wegoff, iMacPro1,1, MacPro 7,1 - everything - Just black screen and reboot. Tried CSM, Get 3 vs Gen4, etc
System worked flawlessly with...