Sorry, my initial misstatement, so there no XenApp involved here, but this is perhaps still of interest.
We are doing some testing with a GRID K1 in passthrough mode with a dental hygiene application that requires DirectX and is being served via a VM configured as a Win 2008 R2 server to multiple users (sorry, the package isn’t supported yet on Win 2012 R2) and it seems to be working really well. Have not worked out the scaling yet, but one Windows server run with 8 VCPUs and 16 GB of memory as a VM on a XenServer on an R720 will hopefully be able to handle 12 or more sessions compared to what would take each XenDesktop VM a vGPU and 4-6 GB of memory. Three concurrent instances barely use together about 1 GB of extra RAM and there is still a lot of CPU power available (it hit around 5%). I’m guessing we should be able to scale this to handle otherwise what would be close to 20 sessions (which would otherwise take 80 - 120 GB of RAM and 40 VGPUs with XenDesktop VMs). We are, BTW, running a XenApp 7.5 instance on the same physical server, also with a GPU passthrough configured, so making good use of the box with a variety of resources.