As of today you can now try out Deus Ex: Mankind Divided with DX12 enabled if you enable beta content in the game. The fifth patch offers several bug fixes but it also allows you to try out DX12 on your system. You can see where to head on your Steam client if you have never played with beta settings before. Once you have enabled beta content and Steam has synced, you will see a DirectX 12 option in the game's Display Options, assuming your card supports it.
For AMD users this should bring performance improvements as long as you first upgrade to the newly released Crimson Edition 16.9.1 driver which you can grab right here. The release notes from Edios/Square Interactive state that 'There are no visual or gameplay differences between the two DirectX versions.' so you will not see new features in your game. The new patch also states that it does not support multiple GPUs in DX12 so ensure you are only using a single card.
We have not been able to test the new patch or driver as of yet, however Guru of 3D did post some quick results in an update to their review of the game. AMD's RX 480, R9 390X and Fury X all see some improvements in 1080p and 1440p but at 4k the card's performance is already topped out. NVIDIA's cards all showed a slight drop at lower resolutions and like AMD, 4k performance is unchanged. The new DX12 patch is not enough to unseat the TITAN X and GTX 1080 as the top performers though.
If you have any issues when playing the game simply disable DX12 and get back to your game. We should see more patches in the near future as the game developers and the two GPU providers work on improving compatibility.