I understand that total war warhammer DX12 is still in beta, but I thought this report might help nip an issue in the bud if it's passed onto the driver team. I've been able to activate DX12 multi gpu and it works pretty well and is very stable , however I have noticed there is an issue with my 295x2 not being able to stay at maximum clocks when crossfire is enabled. I'm currently running the latest drivers (16.6.2).
When crossfire is disabled and I run the benchmark I get decent FPS results like this:
Here is the GPU clocks - you can see by the yellow marker it stays at default max clocks (1018MHZ):
Now when crossfire is enabled, you can see there is great FPS potential at the start but then it drops and bounces around:
And here you can see the issue, clocks don't stay at 1018 but instead bounce around erratically:
I've been talking to some other users on the total war forums, and can confirm at least for someone running 280x crossfire the issue doesn't present. So it might be related to hawaii, over to you anyway. Let me know if you need more information.