Small addition: when I recompose the desktop pool with 3D disabled, the DirectX diagnostics show the same results as on the master image. DirectX then works, but slowly.
Change it from software to "Manage using vsphere client", I believe its available with linked clones as well, I have it on for instant clones and its working. Doing it with horizon may not set it correctly, I saw that before in an older verison.
I set the master image video settings correct in vSphere (3D software, 256MB 3Dram, ...) Then I set the desktop pool to "Manage using vSphere Client".
I created a new snapshot, recomposed and now the VDI is showing the same video settings in vSphere like the master VM.
However, DirectX is still not working. When I check the DXDiag output on both master image and VDI they show the same info, but the lowest of the previous screenshots (WDDM 1.1, Direct3D DDI 10). So with 3D disabled, I got more DirectX features than with 3D enabled?!?