DX11 seems to fix all the stuttering issues. Not sure what visual difference DX12 brought anyway (unless you have an unreleased RTX GPU!).
But now finally after 5 days I can play smoothly on 4K maxxed out with a single GTX1080. Also DX11 gives a different visual look to many of the gfx. DX12 seems to hid some of the lighting or maybe its a glitch but DX11 brings out more depth to the HBAO+ & Volumetric lighting on coloured lighting & water especially.
But now finally after 5 days I can play smoothly on 4K maxxed out with a single GTX1080. Also DX11 gives a different visual look to many of the gfx. DX12 seems to hid some of the lighting or maybe its a glitch but DX11 brings out more depth to the HBAO+ & Volumetric lighting on coloured lighting & water especially.