Crossposting from my other thread for other users seeking advice:
I bought an used GTX 970 (
this one) for the time being, tested fresh and vanilla installs of 10.11.6 and 10.12 with Clover and it works *almost* out of the box with 5k.
SMBIOS iMac 14,2
nv_spanmodepolicy=1 kext-dev-mode=1
Only requirement: The monitor has to be off until OSX boots to the login screen. Powering the monitor on after complete boot shows correct 5k resolution.
If the monitor stays on during the boot process, OSX will recognize two separate screens.
I noticed that with 10.12, once the monitor goes to sleep, it doesn't wake up again, but the system is responsive. No such problem with 10.11. So I'll stay on 10.11 until Sierra has its bugs ironed out.