i watched a video making a performance comparison between a combination of AMD and intel CPUs and a 390X and GTX980, by using the beta version of a game that is made with DX12 support
in DX11, the CPU was a major factor and the 980 had often almost double fps compared to the 390X
in DX12, CPUs did not make much difference in the fps and the 980 got a 2-5 fps increase, while the 390X got from 40 to 120% more fps, getting overall the same fps as the 980 (the margin was +-2 fps compared to the 980 accross all combinations)