Guys, I have a similar configuration, and I have the same problem
(Sorry for english)
CPU : FX 8350
MOTHERBOARD: Asrock 990fx extreme 3
GPU: Sapphire R9 270x Dual x 4gb OC
I have mavericks perfectly work on my pc.
But now i want upgrade to Yosemite (For Final cut upgrade, and other thing)
With 10.10 when I started with -v npci=0x2000 and GraphicsEnabler=Yes the monitor going off.
With 10.10.1 when I start with -v npci=0x2000 and GraphicsEnabler=Yes/No the monitor keep on grey screen with cursor.
If i start with -x the GPU is viewed (Readon HD 7xxx 4096, but is not important this.) and I can also change resolution in 1080p . But this in only safe mode.
I think it is not a graphic problem, because in safe mode the gpu is viewed.
Can you help we?