Depends probably on the games. I can see games like the recently released Stellaris [1] or Star Ruler 2 [2] with it's tens of thousands of spaceships or similar gain the most thanks to the new API's, and it should possibly enable developers to implement things that were previously not feasable because of the draw calls.
Yes, totally forgot that game. Really a good showcase of what's possible.
Yes, AMD profits probably the most from the new API's, the CPU's from them get also a good boost because the kind of single threaded-ness of DX11 [1] hurt them. For example, Hitman with DX11 on a AMD FX 8370 and GTX 980 Ti achieves only 41.5 fps while a Intel 6700k reaches 72.5 fps. With DX12 the same AMD system achieves 65.2 fps (Intel 68.3).[2]
[1] https://www.paradoxplaza.com/stellaris
[2] https://www.youtube.com/watch?v=6ChMEMQxQLw