Yeah except that I tried just changing AGDP and it had no effect. Basically MacPro6,1 was still getting 'processed' as different than Macmini. I haven't made it any further, I can't get MacPro6,1 with a new board-id to work out of the second GPU.
Old QNIX 27" 1440p display connected to DVI, ASUS PQ321 MST 4K connected to 3rd Display Port and the HP z27q 5K connected to the first and second Displayport... all from the GTX 980. The GTX 680, where I want to connect the ASUS and QNIX, is sitting there basically doing nothing right now, in the 4th PCI-e 16 slot. The 980 is in port 1 PCI-e 16.
What I want to discover the answer to is what about the MacPro6,1 board-id is activating some hidden functionality in the drivers to make both GPUs work, and what is it about when no valid board-id is given that makes both GPUs NOT work (but default to a specific functionality where MST DOES work as you would want it to). I want to combine both scenarios. I really thought getting into the info.plist. I was playing around with the various keys in there, for Config1 2 and 3... Instead of changing it to none or deleting altogether, what happened if I actually tried to USE those configs. So use the SSDT injection to name one GPU GFX0 and one GPU IGPU, then use Config3 (which I believe is for the latest iMac 5K) as a model to force GPU1 (980) to be the 5K display with 'special function' (there's some setting in the plist that looks like it could mean that) and to stop it 'unloading' other GPUs like it does for the MacPro6,1 2nd GPU that is portless.
But none of that worked, and I eventually wrecked the plist enough that it would no longer boot so had to go back to vanilla and then back to MacPro6,1 with an altered board-id to get my 5K back again. It's the ONLY scenarious where MST/5K will work - when the board-id you're using does NOT exist. But again, it's being picked up SOMEWHERE else than just the info.plist, as for me just deleting the board-id from info.plist does NOT activate MST/5K
Which I find weird given you and others have said that does work.
I'd love to know where else board-ids are used to control functionality of graphics drivers. THen i can continue my investigation.