[Mac Hammer Fan]

I bought an PC GPU card for my MacPro 6c 3,33Ghz. It was the Geforce GTX 980.

It had to replace the good old Radeon 5870.
It works unflashed in my Mac under 10.10.5 Yosemite and 10.11 El Capitan and the fans are silent (idle 800rpm). Noise and heat are very acceptable, even when the card is stressed.
The card runs PCI 2.0 in a MacPro 4,1 and 5,1 under Mac OSX and PCI 1.0 under Bootcamp Windows.
There is no bootscreen. You have to install the nVidia web driver and CUDA before you put the card in your computer. An annoyance comes when there is a system update that overrides the web driver and that you could get a black screen. However that is not such a big problem. You can turn on screen sharing and reinstall the correct web driver from another computer. This is a lot easier than replace the card by an old GT 120 or Radeon in the computer, update the driver and reinstall the card.
On the plus side the card draws only 165 Watt power which is great. The card has no need for auxiliary power no matter how hard it is stressed. You have to buy other 6 pins power cables than the ones that come with this card, as these are for Windows computers.
As for benchmarks the results are mixed. There is a significant improvement with games and CUDA based applications. But not everything is great. The Mac OS web drivers can be improved a lot.

The good: 3D is really an improvement.
@ Heaven 68 fps (vs 15 on the Radeon) preset extreme

@ Valley 59 fps (vs 30 on the Radeon) preset extreme

@ Luxmark 2,1 Sala 2760 (vs 763 on the Radeon)

@ Furmark 154 fps (vs 87 on the Radeon) 1024 x 640 windowed

@ Tessmark x64 276 fps (vs 17 on the Radeon) Fullscreen 1920 x 1200

@ Octane Bench score 97,8 (Radeon not supported)
@ Da Vinci Resolve Candle 1 NR node 22,5 (vs 2 on the Radeon)
@ L4D2 2560x1440 Best 180 (vs 80 on the Radeon)
@ Tomb Raider 2560x1440 High Preset 55 (vs 37 on the Radeon)
@ Diablo III 2560x1440 HQ 159 (vs 69 on the Radeon)

The bad: CPU-related benchmarks underperform and the nVidia driver is not as good optimized.
@ Cinebench R15 60 fps (vs 70 with the Radeon and 108 on an iMac 4 GHz 27 inch Retina)

@ Novabench 621 fps (vs 631 with the Radeon and 972 on a Windows computer)

@ Image Validation failed on Luxmark 3,1 Luxball (despite better score 11174 vs 3358)

@ Open GL Extensions Viewer 4.1 629 fps (vs 654 on the Radeon)

@ No CUDA acceleration with Wondershare (this was promised)
@ Passmark Perfomance Test 8.0 under Parallels 11: 3319 vs 3630 on the Radeon

@ GFX Bench Manhattan 3691 fps (vs 3712 on the Radeon) Onscreen
@ GFX Bench T-Rex 3335 fps (vs 3350 on the Radeon) Onscreen

@ Compubench Graphics/Video Processing 0,51fps/2,68fps vs 0,51fps/2,68fps on the Radeon (same result)

@ Batman Arkham City 2560x1440 High 44 (vs 46 on the Radeon)
@ In About this Mac the PCI info gives an error and other PCI slots are no more visible

@ Crashes in El Capitan 10.11.2 with Final Cut Pro X (this is also the case with other nVidia cards)

If you really want a bootscreen, you can buy a flashed version of the card from MacVidCards, however you’ll have to pay a big premium and that’s not worth it in my opinion if you have a MacPro 4,1 or 5,1. Users of a Mac Pro 3,1 are less lucky, there the card runs at pci 1.0 speed.

[Mac Hammer Fan]

On GFX Bench Onscreen test the 980 can easily hit the 60FPS limit (come form my monitor). So that isn't a good test perhaps, here is the Offscreen test:

There the Geforce performs better except for Driver Overhead 2.