Before 10.10.3, to get 5k dell display to work one needed to have nvidia videocard with two display-port outputs, special boot option and powercycling of display after bootup was needed.
since 10.10.3 there is no more need for last step (powercycling display). Your 5k display will boot straight into 5k desktop.
10.10.3 brings 5k support to 2013 MacPro, there is no bootscreen tho. No special bootoptions needed, just plug and play, just no bootscreen.
As for nvidia, you can have bootscreen there, mac edition of gtx9x0 needed (or titan x, gta v is coming).
Dell 5k’s display have two inputs, one is single DP link for 4k output, another one is double DP link for 5k. Connect all three into card, you will have bootscreen in 4k and desktop in 5k.