"Following AMD's lead with Mantle, Microsoft aims to provide console-level efficiency with "closer to the metal" access to hardware resources as well as reduced CPU and graphics driver overhead."
http://www.eurogamer.net/articles/digitalfoundry-2015-why-directx-12-is-a-gamechanger
"The figures demonstrate in particular how under-utilised the geometry engines are on our GPUs."
It seems the fx 6300 get's a pretty good boost too:
"Also note that the FX 6300's DX12 results on R7 260X, GTX 970 and R9 290X comprehensively beat the more expensive Core i3 4130."
But most benefits will come in the future:
"Massively increasing draw calls is a fascinating metric, but it is only one small component of a typical game engine. It's going to take new engines built explicitly around the new API to see real gains in terms of denser, more richer worlds, but the opportunities of the inevitable DX12 patches we'll see in the short term are still exciting"
All of this is also true for the open standart API "Vulkan" (directx 12 competition).