For those twisting the 20%, first of all mantle is more efficient for 4 cores. If developers switched from mantle to DX12, they would net a performance loss. Second of all, AAA developers usually code closer to the medal and achieve more performance than mantle, so these improvements should theoretically only be seen in the occasional B budgets, lower budget titles, and indie development.
Not to derail the achievement, that could be a huge boost for indies! ALAS it has been confirmed that the boost is for the PC and not the Xbox One
PS: This is also why Phil himself said we won't see a difference.
Update: Due to a misunderstanding caused by an interruption in the livestream, the original article mentioned that the 20% GPU improvement was on Xbox One, but Spencer did not actually explicitly mention a platform.
Update 2: The 20% boost has been confirmed to be for the PC.
"DirectX 12: DirectX 12 enables PC developers to have a new level of power and control and is a single API developers can access across Windows devices. As shown with Fable Legends running on Unreal Engine 4, there has been a 20% improvement in performance. And as announced today, Epic is creating Unreal Tournament using Unreal Engine 4 running on DirectX 12, which sets a new bar for visual fidelity in PC gaming."
Source: http://news.xbox.com/2015/03/xbox-gdc-2015
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