Running Windows 8.1 on a mac pro with bootcamp.
Have a pair of AMD firepro D700 GPU's
fresh install of blender 2.79
2.79 gives no option for GPU rendering.
2.78c gave option for openCL - Tahiti (2x)
2.77 gave option for openCL - Tahiti (2x)
I was using 2.78 perfectly fine with GPU setting before installing 2.79
I still have a copy of 2.77 installed which still gives openCL Tahiti 2x option.