We are proud to offer the only video cards that give a true 4K SST boot screen on the cMP. I have noticed that if you are using the Dell UP2715K 5K display that in order to see the 4K boot screen you need to pay attention to the order of the DP plugs.
On the 980Ti and Titan-X, the Main 5K cable goes to the bottom port, with the secondary cable going to the next one up.
On the 970 and 980 I have found that the reverse is true. You need to put the secondary cable on bottom port with the main cable on next port up.
To be clear, the "bottom" port to me is the one on right if you are looking at the card installed in Mac Pro. So, the one closest to the Logicboard. Another way to think of this is if you stood the card up with the gold PCIE fingers pointing down, the bottom port would be...on the bottom.
Some oddball cards may have these reversed, so if you aren't seeing boot screens on your 5K display, try switching around until you do.