Dx 12 was a god send for my last PC i.e. a weak CPU with a decent GPU at 1080P, the majority of games ran far better (except for bf 1) minimum FPS shot right up, overall the frame latency was lower and more consistent, which in return offered a smoother experience. I haven't really tested it on my newer system though as felt no need to, that and I haven't gone back to the older games to see how they run now.
But for games like this, that already run superbly on dx 11 on the of majority of systems, there probably isn't much need to use dx 12 unless you are having performance issues.
Also, it seemed to only really be AMD GPUs that benefited from it, it just harmed performance on systems with nvidia gpus (might be different with the newer GPUs now though)