I need to run 6 30" cinema displays, so they need dual-link.
I was driving a single vision 7870 vision tek card for 9 months or so, worked great. Then randomly, it stopped driving more than 3 ports. I can run any 3 monitors, in any 3 ports, but if I try to plug in a 4th or more monitor, all the displays go wonky randomly blacking and and trying to recycle to no avail. Unplug all but 3 displays, and it works great.
It's beyond bizzare. At first I thought that the card died, so I bought another one, same thing. So maybe its the motherboard or power supply? Really at a loss, but I need to get my 6 screens going.
I see, then this dual 7950 setup should works well, and should be more stable then a single 7870.
Even though I am not sure what will happen with dual GPU to power 6 screen. However, it's clearly a that single 7950 can drive 3 non 4k monitor without any problem. Since the nMP can drive 6 screens. So I think I can safely assume that the there is no restriction in the OSX side but just the graphic card or driver's problem in your case.
If you want to have boot screen, and don't want to do any PSU mod. At this moment, I can't think of another easier, cheaper, but more powerful setup then dual 7950.
Dual 7970 is more powerful, but quite impossible to power them without power mod (or more precisely, only by that 2x 6pin source).
From my study, the 680 also draw more power then the 7950. Also, I don't think you can be easily under volt the card + flash Mac EFI. Since the under volt 7950s are already right at the edge of the power consumption that the 2x 6 pin can deliver, I think dual 680 is also not quite possible without PSU mod.
If no need EFI (or don't mind to pay for it), and don't mind to deal with the web driver, may be dual 750 Ti or dual 960 is also work. But AFAIK, it's not necessary more powerful then dual 7950. With total 4GB VRAM on these 2 maxwell card vs 6G VRAM on the 7950 (to drive 6 screens), and no significant performance improvement, and no free EFI option, and no native OSX driver support. I personally don't think this is a good option.
Anyway, if you want to know more about my setup. Please feel free to tell me. However, I cannot run the 6 screens test for you.