I've spent the last week trying to get a Windows 8.1 guest up and running with GPU pass through. I've finally got it working and now I've noticed slow network performance (i think its always been slow). I'm getting 1MB a second throughput. I have the HVM drivers installed are there any other tweaks i can make to improve network performance?

You need to disable tcp offload. Not sure if just on the virtual network adapter or if you need to on the physical in unraid too. Try virtual first and see.

I found the fault, not sure how i hadn't noticed it before but my switches ethernet port was set to auto and was connecting at 10mb  i forced it 1000mb and now were back to full speed!