I have the Dell 5K display running at 5120x2880 60Hz on the nMP using the dual mDP to DP cables that come with the display. It exhibits the Retina effect scaled to 2560x1440 -- identical in quality to the iMac 5K sitting next to it.
Also works with the 2010 Mac Pro tower with the Geforce GTX 970, 980, and TITAN X.
Wish Apple offered their own version of the 5K display. I have to say this is one of the nicer looking Dell displays with height adjustment and option to pivot to portrait mode.
The dual cable approach is pretty unclean. I don't see Apple coming out with a 5K Display until Thunderbolt 3 and they update the Mac Pro alongside it.