I've been on a mission to get this screen working on my 4,1 Mac Pro. First I tried the 7970, no dice, then bought a K1200, which I think would have worked had I added the `nv_spanpolicy` boot arg, but I forgot. Then I bit the bullet and bought a GTX970, set the boot args and the 5K screen came to life but all was not well. Firstly the screen seems to have a yellow tint at the top and I'm getting screen tearing on anything that has to redraw a large percentage of the screen (e.g. a modal window on a website with a backdrop). Anyone else getting this? I thought the iMac 5K GPU was meant to be under-specced but I must say, never seen any graphical issues in day to day use.