My 5 box setup took a bit of a performance hit as well, still using DX11. Everything is still playable but there is some stutter now and then.

As per Google's instructions I ran dxdiag and it says that my version is DirectX 12.

So obviously WoW thinks my system is not capable of DX12. I am running an i7 which I though had hyperthreading, wonder if I should just try DX12 and see what happens.