I had the Nvidia GTX 680 2GB for Mac and it was a pretty good card for HD video editing in Premiere Pro. But when I began editing UHD footage it became a bottleneck. Render times, export times, and playback within PP were painfully slow. I tried the Quadro K5000 4GB and it wasn't any better. Incidentally, around the same time I got a Dell UP3214Q UHD monitor. I was unable to get the Dell to display UHD @ 60Hz with either the GTX 680 or the Quadro K5000. So I got a MacVidCards EFI-flashed Radeon 280X 3GB which was much better than the GTX 680 in terms of rendering, exporting, and playback within PP but I still could not get UHD @ 60Hz on the Dell. So I bought an ASUS PQ321Q UHD monitor... still no 60Hz. So I returned the ASUS and sold the Dell UP3214Q (it's return period had expired) and got my Dell U3014 2560x1600 monitor.
The thing with UHD monitors is that it's still a nascent technology. Most of them require Multiple Stream Transport (MST) which segments the screen vertically down the middle into two halves. This is a stop-gap technology on the road to Single Stream Transport and it doesn't work very well. There are a few UHD monitors out now that can handle SST and I would recommend getting one of them. Just make sure that your graphics card can handle SST. The only one that I know of right now is the GTX 970 or GTX 980. I have the 980 and it's 2x faster than my old 280X in terms of rendering, exporting, and playback within PP.
I hope that long story is helpful.