It doesn't seem that XBONE would have that much improvement with fully mature DX12 code over the older code, as I'd be willing to bet that most of the older code already implements most of the significant improvements that DX12 has over DX11 on Windows, as most of the code would already be designed to be closer-to-metal than the ordinary commonplace Windows DX11 code.
Ok with science that the big bang theory requires that fundamental scientific laws do not exist for the first few minutes, but not ok for the creator to defy these laws... 