http://www.theverge.com/2014/1/23/5338162/microsoft-q2-2014-financial-earnings
That sure sounds like a company in dire need of a culture change. Anyway, in GPU limited situations there have been several hints that the gains will be around 10-15% for Mantle use in BF4. (This was mentioned by an AMD rep to computerbase.de, FWIW) I certainly don't see that as game-changing, that would amount to a rough 5-6 fps gain at WQHD resolution assuming the baseline framerate is around 60. (2560x1440) That would not be an amazing gain.
That is speculation based on what an AMD rep told computerbase, but the bottom line is we need an actual BF4 Mantle product for reviewers to test, and we don't have that yet. So if those are the representative gains for GPU limited situations with Mantle? Then I don't buy into the theory that DX is going to be "doomed" as has been mentioned by a few folks - it isn't like MS is going to sit back and let DirectX fall to the wayside. DX will be just fine, and it will evolve over time just like OGL has.
From what I recall, MS is in fact making some adjustments to the abstraction layer for a future version of DX. OpenGL already allows more direct GPU communication similar to Mantle, as well.