Just found out
this card works really well in the cMP. (Off topic a bit, this is the R9 280, NOT R9 280X)
2 x 6pin, so no power issue.
Very good cooler, I leave it at the stock 800MHz, and lower the voltage to 0.981V, with stock fan control. Max 67C in Furmark (33C ambient), and virtually zero fan noise under full load (the PCIe fan will spin up a bit, which is just audible, but already louder than the card. With stock 1.09V, the card will go up to 72C in Furmark, that means most likely won't go above 70C in normal ops. Also, this cooler will push all the hot air out of the case, but not circulating inside the case like most of the PC card does.
Reference design, same device ID as the 7950 Mac Edition, 2 x mDP which is good for my Apple ACD, and super easy to create EFI ROM for flashing. (Standard dual ROM design, fail safe for flashing card).
No need to do anything for the driver (this is one of the reason why I didn't go for the nVidia at this moment).
$150 new card. I know it's not the cheapest, and lots of new card better than this now. But I really can't find another one has all the above advantage with this price (at least not in my town). This card is nowhere near the top of the line, but really a decent choice for the cMP if it fit your need.
N.B. I think this card has few difference versions. I pick the one that stick to the original 800MHz clock speed. Some other version may come with stock OC (most likely have 8pin input for OC, which is not ideal for an unmoded cMP), since I don't need those extra power most of the time, and I know how to OC my card anyway. So I choose this non-OC version, but a OC capable design, which will allow the card run cool and quiet in normal setting.
Update: PCIe 2.0 mod completed (R17 removed)
I simply use a tiny screw drive and apply some force back and forth few times, then the R17 pops out in 30s. The process is simple and much easier than I expect. There is no R17 marking on this card. However, it's at the exact same location as the reference HD7950. The only caution is that DO NOT apply too much force. The join is not that strong, if you use too much force, when the R17 pops out, you may unable to control the screw drive and damage some other components.
This is the actual screw drive I used.
No R17 marking, but exactly the same location and layout as the reference HD7950.
The R17 pops out and fly away, I have no idea where it is now.
OceanWave confirm that the card is now running at 5.0 GT/s range.
Update 2: Review
It's about a year I've use this card now. The card still works well. Always stay quiet and cool (0.956V). I've seen very minor graphic corruption like the VRAM has trouble twice (random single small block of square on the monitor in difference colour). However, both time fixed by reseat the card (plus some cleaning by comprised air).
The down side. This card is flashable, however, When boot with Mac EFI ROM, the HDMI port won't work. To me, this is actually a bonus function, because I can now use that switch to control if the 4k TV is considered connected by the OS. When I don't need the TV, I can simply boot with Mac EFI. So, I don't need to unplug and cable, but the OS won't detect the TV, therefore my mouse cursor cannot go "offscreen". (So far, I can use an apps to virtually disable a monitor, but OSX cannot totally disable a connected monitor like Windows does).
For crossfire, Windows 10 and the most up to date AMD driver can boot with Mac EFI, but still crash at some stage. So, must boot without EFI in order to enjoy crossfire.