Some people on the OCUK forums were saying they got slightly better performance on DX12 with a 1080.
DX11 maxed out settings including HBAO+, HFTS etc... GTX 1080
DX12 maxed out settings including HBAO+, HFTS etc... GTX 1080
And then this is my own testing with everything maxed/turned on i.e HBAO+, HFTS etc... at 1440P DX11 and DX12, DX12 giving quite a nice boost with the GTX 1080 -