In case anyone did not noticed, then the main speed-up in DX 12 slide is, that in the DX 12 case, there is missing the KM mode driver - eg. the Windows Display Driver Model (WDDM). That come up with Vista, restricting some possibilities that WinXP do just well. As such, it was hated and therefore since WDDM 1.0 in Vista we get envolved a bit. Windows 7 have WDDM v1.1 with some functionality restored and Windows 8 and 8.1 come with version 1.2.
Now every these versions does REQUIRE the Kernel mode driver, as we know it. Therefore there is no way the nicely looking graph can run on Windows 8.1, as it will require announced but never produced WDDM v2.0 and 2.1 as step, that is according to M$ targeted to "future windows."
M$ was talking about WDDM v2.0 on WinHEC 2006, but since then nothing was changed. Many expected new WDDM in Win 8/8.1, however nothing materialized. There are only slight evolutionary changes to v1.2, nothing that can allow direct access to the metal like Mantle do. Not to mention the overhead problem…
There is no way to bypass this using current WDDM v1.x ways.
However the graph (in first post) is not showing the KM-driver and DXGI (Microsoft DirectX Graphics Infrastructure). In Mantle, things go after the Application thru just the User-Mode Driver to hardware, eliminating the overhead of OS.
Even AMD and nVidia mentioned that DX 12 will require "new" driver model, however no-one is explicitly saying, that w/o WDDM v2.0 the DX 12 cannot run." But we can assume, based on the informations given, that this is the case.
From that perspective it looks like that for the Vista/Win7 is the DX 12 almost hopeless (while Mantle works just fine) and it is a good question, if M$ is willing to implement WDDM v2.0 into Win8/8.1 - that would require plenty of work for no gain. And mainly - since Win9 is planed at April/May 2015 (and come with WDDM v2.0 and Start button), then what is the better sale argument for new Win, that faster DX 12 with less overhead?
Many users hesitate to upgrade even todays from WinXP (for many is WinXP most their computer can handle anyway), so M$ is likely to go this way, witch would, in the end, play well into cards of AMD and Mantle.