I finally completed a project I've been thinking of doing for several years,
Attempt using KVM with VT-d and passthrough PCI devices directly to the virtual machine to see and evaluate exactly how useful it is.

My machine in this case is a Intel i5 4460 with a cheap MSI motherboard (that luckly supports VT-d)

I had an AMD r7 250x that I bought some year ago for a totally different project laying around.. actually for my PowerPC G5; I never successfully got the entire 3d stack to work however so I gave up on it now, so once I got KVM installed and was about to insert the 2nd graphics card I noticed this motherboard only had one PCI x16 slot and that slot was occupied by an Nvidia GTX 750Ti that is natively used by linux.
Got a very cheap PCIe x1 to x16 adapter from ebay. I believe it was like $4; installation was easy- it even fit inside the case so it doesn't look any different even though using the adapter.

Installing the card etc was just plug n play, essentially just had to set nomodeset for the radeon driver, add the pci device path to the vm configuration and boot up windows, install the catalyst drivers and it just magically worked.

This is of course not the quickest card around, I just installed some various games to demonstrate that it "just works" regardless of OpenGL, DX11 or DX10 etc.

The most difficult thing to actually get working properly was audio, I use a focusrite USB soundcard I had laying around and it was a lot of problems to even get it to start output sound- USB Passthrough did not work at all, it just hung windows on the login screen once the focusrite drivers was installed,
as a last resort I thought I export the entire USB controller as a PCI device instead- and that worked, with the caveat that I loose every USB device on the linux installation once I boot the VM... I can live with that for now though as I use synergy for both the linux and the windows vm anyway.

Additionally the audio is a bit distorted at times, I haven't figured out why yet- originally I thought it was that I overdriven the signal between the soundcard and my other soundcard, but as it can be heared in this video it actually distorts before it leaves the soundcard at all.

And finally the screen recording apparently dropped frames at times especially when I loaded the machine significantly- this was not the case when using the machine in person it's virtually impossible to tell apart from a physical windows installation on the same hardware.
...
https://www.youtube.com/watch?v=CzxKxRtEZHM