I was able to play deus ex mankind divided a high details thanks to the DX12 port on a very old i5 750 (though overclocked at 3.8GHz) and a R9 280X. In DX11 mode it was unplayable. A good DX12 port can significally lower the CPU overhead, which means you will see benefits only CPU limited scenarios. If you want more, the developers have to re-thing completely the game engine and the rendering algorithms, not just making a port adding a D3D12 backend..
You should play Deus Ex on a nVidia card. DX12 is 33% slower in cpu limited scenarios than DX11.