To make my Apple mid 2012 Mac Pro Metal compliant I purchased a new Radeon Pro WX 7100 with Display Output Support (@ 60Hz):
4 @ 1920x1080
4 @ 3840x2160
2 @ 5120x2880
1 @ 7680x4320
I used the cable that came with the card, Display Port to DVI-D connectors to connect to the monitor.
Good news is that it worked, computer started up perfectly, in fact faster than the original AMD graphics card that came with the Mac Pro computer.
I opened the system prefs and I can see the Metal label under the System Report, signifying that I can now upgrade from High Sierra to Mojave and beyond.
Detail of System Report | Metal: Supported, feature set macOS GPUFamily1 v3
The problem is that I am only getting 1200 by 800 resolution.
My monitor is an Apple 30" Cinema HD Display with the capability of 2560 by 1600 resolution, which was the resolution prior to this new card. According to the Radeon Pro WX 7100 specs, it should bring the resolution to at least the maximum of the 30" Cinema HD Display.
I believe the problem is the cable, that it is not providing the DVI-Dual Link ability. Whe I purchased the Radeon WX 7100 from B&H Photo it was suggested that I purchase a j5create Mini DisplayPort to 4K DisplayPort, HDMI & DVI-D Adapter.
The problem with this solution is that the adapter is a Mini-Display port connection, while the card has 4 full sized Display Ports.
It always seems to come down to two expensive devices on either side of an unwilling or incompatible adapter. Over the years, Apple's port churn has not helped. :-)
Can anyone suggest the proper adapter? I am so close, I havbe this great new graphics card and a beuatiful display but they are not talking to each other.
This link is from the Apple Forums, in it a gentlemen details part of the problem but not what device might solve it.
PS I am new to the Radeon forums, if there is a better, more appropriate subject area, please let me know.