I have just purchased a new 27 iMac retina i7 4gHz with 16gb ram & AMD Radeon R9 M295X 4096 MB video
I now find that when making edit adjustments within the develop module the picture screen goes black when I move the adjustment sliders.
The machine meets all the required specs for using the graphics acceleration mode, but there seem to be an issue that I cannot permanently resolve?
I can fix temporarily by toggling the graphics acceleration off & back on again, but this is far from ideal!
It probably a combination of the driver Apple is supplying for that GPU and the simple fact the GPU acceleration system in LR isn't really ready for prime time.
With that system, the specs of it, just turn that GPU feature off and be happy.