The resolution doesn't matter, but the EFI just doesn't support DisplayPort 1.2. You should see a boot screen when connecting the screen through HDMI or DP 1.1.
Thanks for the advice Fl0r!an as always you are correct. I didn't think of trying to switch it to Displayport 1.1. Working now at 30hz with the boot screen visible but only stable at 30hz at 60hz it constantly goes black and comes back.
HDMI Port 1 (HDMI 2.0) also does not work at 3840x2160@60Hz like it should but will give a distorted boot screen at 30hz. Thanks for the advice on the 1.1 looked everywhere on various forums and no one made that so obvious point. (Sometimes I have moments of Genius other times I'm dumb as rocks. lol)
vinnie1978 I appreciate the obvious comment, I know this monitor isn't supported, heck my MSI R7950 is not Apple supported either. That is why we are all here to give power back to the users and mod things to work on Apple. I purchased the monitor for $179.00 no tax and free shipping it was SST and for the price was worth trying. Also would not be the first time I purchased something that was not supposed to work for my Macs that I fiddle with until I got it working. Just Sayin'
Now, while browsing the various forums I did find that some PC users changed the profile in windows [C:/users/user name/AppData/Local/ATI/ACE/profiles.xml] to keep the memory clock speed at maximum to avoid the glitch of the screen flickering on and off. Wonder if there is a way to do that in macOS Sierra or even better on the actual rom of the card. Maybe it's time to get my Hex Editor out. This card has two Roms anyway.
***
UPDATE NOTE: So I did a PRAM Reset and then the mac would no longer boot would stay in the same boot loop as when in Displayport 1.2. Had to restart with PC ROM and reset everything to default 60hz, Displayport 1.2 restart then set it back to Displayport 1.1 and 30hz to get the boot screen again. Simply Weird ***
