Double the API performance doesn't mean double the system performance.

Consoles have a multitude of API's for which they can use, depending on how complex the game is, higher level API's (Like Direct X) generally are slower, however they are also very easy to use and develop for.
Lower level API's however you have to get a little more dirty with, but does bring with it some good performance benefits.
Then you have the very rare (Especially now-a-days, with a couple of exceptions.) where it's actually written to the metal, skipping the API and other software stacks completely.

Basically, from Microsoft's press releases, information and outlines of the improvements Direct X 12 is going to bring, the main goal is to take some of the low-level API's advantages and abstract it in a way to keep developers lives easy, this is the goal for not only the Xbox One but the PC version of the API.

The kinds of improvements should be a little more severe than what Mantle gave us, AMD's focus was mostly on draw cell improvements with that API, but it gives you some idea of the direction that Microsoft is heading down.

The part I am looking forward to is now that Microsoft is fealing the pressure from the Playstation and AMD's Mantle and a reinvigorated OpenGL, they are no longer sitting on their laurels slowly iterating in the PC space, which pleases me greatly.

Hmmm...Interesting read. Thank you for this.

I checked the Neogaf thread and the first thing I saw was my comment, almost word for word. At least give some credit, man. 

I also dont agree its x2. Thats exagrated. But along with Phils comment its like PDZ - Halo 4 there is a significant performance gin using DX12. 

This isnt some secret sauce, like some chip not switched on inside the xbo one. This is a PROVEN enhancement to the direct access and control over the core of the hardware. When all is sai and done this is good news for XB1 in terms of great story visuals and cinematic gameplay. There is a dfinite performance gain using DX12 over 11 for Xbox One. First party tites will be benefit mostly as they only have to build one version of the game. But 3rd parties probably for a couple of years will still choose the easier option of DX11. Some like Ubisoft or EA wit more funds may dabble into DX12 though. But I expect Forza 6, Halo 5 etc to be sporting the better DX12. Maybe even Quantum Break will have beeen shifted over to DX12.

Take it as a sign of flattery that someone thought you were incredibly witty. :)

Have you even tried "that other one?"

It's not good to make fun of the OUYA without ever trying it.

That seems to be the generic reply to my posts these days on this forum. :P