Basically when you run a bench on DX11 with AMD graphic card it's obvious Drivers are consuming lots of cpu.
On the Opposite with a Nvidia graphic card, DX11 drivers are so well optimized that they do not introduce the same amount of cpu overhead as AMD drivers.
AMD has simply never been able to provide optimized DX11 drivers for their graphic cards while Nvidia is providing very well optimized drivers for DX11 and DX12.
This is why There's no improvement while moving from DX11 on DX12 with a Nvidia card.
I have 2 GFX cards (Nvidia & AMD). Basically My GTX770 outperforms my 290X on AOTS with DX11, while anyone would expect the opposite.
On DX12 my NVIDIA gfx card performs as well as on DX11. And my 290X Gives it's full potential in DX12 (outperforms GTX770 which is of course expected).
Do not expect any major improvement with current Nvidia cards on DX12, since anyway, Drivers are not putting a huge overhead, and this is for this specific reason that Mantle / DX12 have been introduced. They just Help AMD GFX cards to perform better. Nvidia is simply very good at optimizing drivers on DX11 / DX12.
Now my 2 cents that if Nvidia put a new GFX card on the market with an architecture specifically adapted to DX12, it's definitly going to outperforms AMD GFX cards.