I have the MVC 980ti and the Dell 4K P2715Q and I see the boot screen and it is working fine in OS X and Windows 10 on a MacPro 5,1 2012. I am currently running the 4K monitor from DP, and the secondary monitor from a DVI port (since the cable was already routed through my desk).
The only issue I have, which others have also reported, is that once in a great while the monitor will not come on from computer sleep. Pulling and replacing the power cord causes the Dell monitor to reset and it works again. I know OS X can see the monitor is there, as I can blindly login and my second monitor (a Dell U2713HM) will switch from the lock screen to active display. If the primary monitor was truly "off", the login screen would switch to the secondary display, as it temporarily does when the power cord is removed and reinserted.