same issue
yea I have the same issue but I'm using a EVGA GTX 980 SC i tried the thunderbolt firmware update but it told me that my machine did not need it and closed the installer. I'm currently using Mac Pro 1.1 with 10.10.1 installed with SFOTT and i had x2 EVGA GTX660s SC installed in it and had no issues with HDMI support. only the DVI and 3 display ports are working but when I use windows 8.1 in bootcamp the HDMI works great even when it comes to upscaling to 4K is there a kext that could be modified in anyway kinda just spit balling here 😉
My Mac Pro 1.1 Spec's : x2 intel Xeon x5355, 16GB Kingston FM RAM, 120GB PNY SSD,1TB WD black Enterprise HDD,900W power-supply,EVGA GTX 980 SC