OK , I'll add my 2 cents worth here .
Add two RX 480 8GB Reference Blower Type Graphics Cards to the cMP 5,1 . They take up a total of 4 PCIe slot inches . The Mac has five slot inches , so you'll have one thin card left to install here ...
Yes , this Mac will do it . Each card has only one standard 6 pin PCIe booster power cable requirement and this Mac's logic board will be happy to provide the connections . Just get a couple power cables .
And Mojave will run . You can even install it with these . These cards perform around just 12 percent slower
each than the official RX 580 . But there are two of them installed , so performance is actually much higher .
What you won't get is that pretty Apple logo at start up , boot picker , single user command line and any other esoteric EFI function you
simply must have .
What you will get is recovery mode and the cheapest dual GPU acceleration that programs like Resolve and FCPX crave .
You'll also get tons of DP ports . You won't get that with either official brand new Mojave card .
Make certain that fan hole is unobstructed as best you can - it sucks in .
Total cost = $200 for the pair .
They look like this :
You should rebuild them as used cards off of eBay really need it , due to crypto mining and such .
Good Hunting .