So I recently upgraded to two Flashed MSI Twin Frozr R9 280x's in my 2010 Mac Pro from a single OEM 5870. I was mainly looking to improve render and export times in FCPX, and I went with Dual R9's because the chip architecture in the R9 280x is very similar to the D700 Firepros in the nMP. I've had the cards installed for about a week now and they are running fine. (Boot screen on every restart, no kernel panics) however I did not get the speed increases I was expecting. I have both cards installed in x16 pcie lanes in a cubix expander and I'm getting comparable render and export speeds to my single OEM Radeon 5870 in final cut pro. I'm even getting the exact same benchmark scores in Cinebench and Unigine Heaven between the R9's and the 5870. The Cubix doesn't have a bottleneck. I installed both cards inside the Mac Pro (in the 16x lanes) and I'm getting the exact same benchmarks. What's weirder is that one R9 card is consistently getting benchmarks FPS speeds that are 10 frames/sec slower than the other R9 card and the 5870. (I booted the Mac at different times with each card installed separately to compare.) I am getting between 60-65 fps speeds in Cinebench and Oceanwave on one R9 and my 5870, and between 50-54 fps on the slower R9 card. I know people that are getting speeds above 90fps with the R9 280x on OSX. So strange. I thought the cards might be underclocked but I checked their clockspeed in Oceanwave and they are actually running at their "boost" speed of 1020MHz. I have run out of trouble-shooting options. I even did a clean install of El Capitan on a brand new SSD. Does anyone out there have R9 280x's installed in their cMP? What kind of speeds are you getting? Any idea what the issue might be? My machine's specs are: 2 x 3.46 GHz 6 core CPU 32 GB 1333 MHz DDR3 1 TB Crucial SSD Boot Disk.