Two of the cpu cores are reserved for the operating system and kinect. DX12 is a API anyway. Mantle surely will drive the GPU more directly and improve performance. AMD know their hardware a lot better than Microsoft do;
http://hothardware.com/News/AMD-Mantle-vs-DirectX-Benchmarks-with-Battlefield-4-and-Star-Swarm/
Clearly Microsoft are fighting hard to blur the evidence that the ps4 is more powerful, they are getting developers and anyone they can influence to make the case the xbox one is competitive in performance but clearly they can't do that today because its clearly performing below so they try to make the case that in the future the xbox one will perform better.
If you want to see how powerful a console is just see how it performs on its best performing games of today on the type of games you like, be it fps, race games or whatever. There was a case to be made with ps3 and ps2 that they would need time to understand before the potential of those consoles came out and in both instances it was correct.
