A exec says he wants all holiday 2016 Frostbite games to have a minimum spec of windows 10/dx12. Says he knows it is an aggressive step but the benefits are worth it.
Asked wether it will benefit Xbox One games he says "Yes".
I don't think anyone is disputing that DX12 will provide SOME boost to Xbox One games. The Xbox One has been getting regular updates to it's version of DirectX though since launch so that's not at all unusual. The Xbox One APIs are contantly being tweaked they don't just update with a new release of DX so the DX12 update isn't much bigger of a deal in terms of performance than any other XDK update. (It's a bigger deal in ease of portability between Windows and Xbox One though).
Here is a rundown of some of the frequent changes the Xbox One has already gone through: (as of Jan. 11 2015)
This evolution of the XDK will likely continue after the DX12 release as well (independent of DX12 updates) and Sony is no doubt making similar updates to the PS4 APIs (GNM, GNMX) behind the scenes. This is normal for any console as the API's continue to be improved and refined over the life of the console. We only know more about the MS one because it was leaked and Sony's wasn't.