DX12 should roughly double the CPU capabilities on systems limited by CPU issues on a modern rig, but if your GPU happens to be the limiting factor, you'll see quite a bit smaller benefits from DX12. Also DX12 GPU need to be very new to get all the benefits, as I have a bad feeling my GTX970 will only be compatible with the DX12 APIs, but not the full feature set. In any case, you should be able to get at least some kind of stable fps on your rig, but do use an AMD optimized version as suggested before.