I got quite massive minimum fps inceases overall. Average and Max fps didn't really raise at all. Overall , a lot better perforamnce under DX12. Also stutters have "gone" (need more gametime to be sure) on my Fury X, and could even increase details.
Well DX12 certainly fixes all overhead problems from AMD driver on those streets. They were the real "big" issue. Only real problem that remains on all ultra now on my Fury X i s that 4GB of memory. This game really uses it a lot. AMD driver does use a bit over dynamic memory, but I feel they could use it even more. Well what do I know.
Here's the 2 pictures to give you idea about the overhead caused by DX11, and the lack of it at DX12: (DX12 pic is with a bit higher settings, and from a bit different location (but suffers from similiar overhead in dx11 that this comparable picture):
DX11, and AMD driver overhead:
DX12, no AMD driver to slow things down :
Ps, take a look at ram usage of DX12 on ultra settings