Ok so...then if no one's actually taken advantage of it...that's the point I am making. You would obviously begin seeing an improvement on both. Because now it's available on PC, you are likely to see this increase in performance that has laid dormant on the X1, on both platforms. Now multiplatform publishers have more of a reason to use that, since at least 2 of the 3(sorry Nintendo) are compatible.
You're misunderstanding what people are telling you. DX12 is meant to give PC developers the same kind of environment console devs have always had. X1 already allows low level development.