There's been a lot of great articles taking a first look at DirectX 12 performance via the Ashes built in benchmark.
Here are a few:
PC World: http://www.pcworld.com/article/2971612/software-games/windows-10s-radical-directx-12-graphics-tech-tested-more-cpu-cores-more-performance.html
Regardless of the video card, DirectX 12 trumps DirectX 11. The long story short of this is that DirectX 11 serializes commands from a game on the way to the GPU. DirectX 12 does not. This means that games that fully utilize multiple cores to talk to the GPU (like Ashes) will see pretty significant gains.
Nvidia mistakenly stated that there is a bug in the Ashes code regarding MSAA. By Sunday, we had verified that the issue is in their DirectX 12 driver. Unfortunately, this was not before they had told the media that Ashes has a buggy MSAA mode. More on that issue here. On top of that, the effect on their numbers is fairly inconsequential. As the HW vendor's DirectX 12 drivers mature, you will see DirectX 12 performance pull out ahead even further.
As some may recall, it took a long time for DirectX 10 to pull ahead of DirectX 9 in terms of performance. By contrast, DirectX 12 starts substantially ahead of DirectX 11. It's a huge win for PC gamers. And the next-gen cards from AMD and Nvidia will make that gulf even larger.
If the DirectX 11 vs. DirectX 12 results also seem to make clear: The quality of your drive makes a huge difference. You really can see this in DirectX 11 results.
Feel free to add links to other benchmark posts.