Hey, great news. I just finished my first final El Capitan install on a 5,1. I was able to leave my EFI 980Ti in the whole time, the key being to first use the:
nv_disable=1
in terminal. The EFI drivers give fine video output, even to a 4K display (SST) to allow the install to run.
Had another user report same thing in our blog. And it gets even better, if you were running the GM with most recent Web Driver and install the Final build over it this way, all you have to do is then turn the Web Driver back on via terminal and reboot. must either clear NVRAM or actually type:
sudo nvram boot-args=nvda_drv=1
If you use the Nvidia driver panel it will ADD to the boot args and leave the disable in, you need to remove it.
The b07 Web Driver just looks for "15A" as build number, so any build of 10.11 should match.
The 5K nv_spanmodepolicy=1 flag can be added as well for 5K to work. Still have to power cycle.
I am sure Nvidia has a final driver coming soon, but nice to know that you can use El Cap on Day 1.
Thanks to Nvidia and Netkas we have wonderful options for cMP.