I'm running 6 30" apple cinema displays with this card:
http://www.amazon.com/VisionTek-Radeon-GDDR5-MiniDP-Graphics/dp/B0085O90SQ?ie=UTF8&keywords=7870 6 display&qid=1459492059&ref_=sr_1_1&sr=8-1
This works in a 2010 Mac Pro (5,1) with no modifications. It boots up with no boot screen, but the screens popup on login in. I'm running this with 6 30" Apple Cinema displays. They are all plugged in with the the apple mini displayport adapters. So it's a big step up performance from my two 5770 cards. All 6 screens work. Metal/OSX acceleration full supported.
Two issues.
1) When restarting/rebooting, one or two screens will often stay black and not come on. Same when the system goes to sleep. So when the system goes to sleep, the easy way to 'nudge' the sleeping display back awake is to hit COMMAND-OPTION-EJECT which will put all screens to sleep, and then waking them again will often nudge on the sleeping display. Bit cludgy, but not a big deal once you figure out how to do it.
2) Once the system is up and running, the fan on the video card can kick up when the system really isnt being stressed. For example, if you use the trackpad gesture to show the desktop and hover all the windows half way before they are off the screen, you will hear the fans kick up until you release the gesture. The fix is weird. If you run a 3D game like Elder Scrolls online, it apparently initializes something on the card, and thereafter, the fans act normally and don't kick up. The card runs super cool even when pushing it. You get pretty good frame rate, for example, on Elder Scrolls online even with max detail on the 30" cinema display, no fans kick up.
Realistically, if you want a decent speed video card, that supports Metal acceleration on the mac, and supports 6 displays all on one card, this is your only choice. Other than the two annoyances above, it's pretty much totally trouble free and zippy. Not bad for a PC card on a mac.
It continues to work on El Capitan 10.11.4, even after the updates, with no problems.