I wonder how many DX12 games are true DX12 games and not DX11 games converted with a DX11-to-DX12 compiler? And how do you tell if a game runs better, when DX12 is used to draw more (timy) objects/make more draw calls, but all things combined result in the same frame rate as the DX11 version?
It's like I tend to say: Hardware (API) designer giveth, game developer taketh away.
If you would take a game like Arma3, you would expect a DX12 version of it to have bigger viewing distances with more objects. But then the dev could select twice as many different trees and twice as many different buildings. That would not change the view of the landscape at distance that much, but up close you would see less repetition of objects. This choice would be the trade-off: better performance or better looking.
If DX12 has the potential to give 10-20% better performance, it'll likely be used to show more on screen and not for more frames IMHO. Still, it's good that PCs can now have console-level draw calls.