Does the GTX680 run 4K @ 60Hz? Does the dual-link DVI support 4K @ 60Hz? The site says 2560x1600 max.
AFAIK yes, 60 Hz with DisplayPort if the display is connected via SST: https://forums.macrumors.com/threads/apples-remaining-surprise-4k-at-60hz.1668673/page-4 But no boot screen.
No, the dual-link DVI port does not support 4K. DVI supports up to 2560×1600 Pixel ad 60Hz.
The GTX 680 that Synchro3 and I were discussing IS already a Mac Edition and would show the boot screen.