No Metal GPU Acceleration on MacPro D500's

Does anyone know why a MacProd with dual D500's would NOT get GPU acceleration from Metal Graphics.

AIUI the AMD Metal driver (Although information is pretty sparse) should provide for hardware acceleration. However my Retina laptop with Nvidia Discrete graphics (Late 2013) runs rings around by MacPro 2013 with dual D500's because it would appear that the D500's are NOT being used to accelerate graphics.

I thought it was my own code, but grabbing the metal Basic3D example and running it reveals that no GPU acceleration would appear to be taking place. While my laptop with simple i7 uses about 30% CPU the same app on my MacPro (With lower resolution) uses 130% CPU (And Xcode says 0% GPU too).

Anyone got some clarity? I raised a bug on developer.apple.com last month but it's so far just been ignored by Apple...