Just fired up the Dell UP2715K and can confirm this is still able to do 5K and Retina HiDPI resolutions.
The Terminal Command is still:
sudo nvram boot-args=nv_spanmodepolicy=1
Problem is, you need to have SIP disabled to enter this.
Good news is our EFI cards all let you get to Recovery Mode to disable this.
Just Hold "Command-R" (we used to call it Apple-R when we thought different) at boot, the Recovery Partition is no longer a Partition, and will not show as a Boot Option
Under Utilities is Terminal, pop the baby open and type
Reboot into HS and run the above. Still have to power cycle the UP2715K to get the 5K to kick in