I can pass a gpu (R9 270) successfully to a windows guest. After installing the AMD driver (but before a proper reboot), windows device manager sees the GPU correctly and graphics appears to be accelerated on the GPU.
Although, after reboot (after installing the driver) I always a BSOD when passing through the GPU.
If I remove the passthrough GPU, it works again. 
I guess this is a driver issue? Any ideas on how to fix this?