A few weeks back I was playing with cabling gpu's and testing things...I borrowed a quadro FX1500 I usually don't use for pass-through to VM's. I added a test layer to a xp vm install I knew I had built in the drivers ages ago, and completed my test. I then noticed different behavior from my typical AMD driven VM...Well that percolated around until I thought of saving my second build from failure, I'll avoid the long story...SO, I put a K600 in my X9SRA where the AMD failed to pass and it's fantastic.
And the punch line, back to that different driver behavior stuff. With the genuine quadro drivers the VM's primary QXL is exactly that, #1. With my AMD setup windows will switch and leave a dead qxl. Anyway, when I fire up a opengl window that should fail on the qxl window it doesn't! The nvidia gpu shows load, clocks up and accelerates the QXL!!! WOOHOO!
Yep, after 2 years I finally hit the combo. Both the qxl window on the host and the passed feed to a separate monitor are usable simultaneously which solved so much. On another screen I do pass an amd to a debian child, I haven't yet tried both hardware and software screens, but soon!