During GDC 2016, Microsoft revealed a list of games which use, and will be using the DirectX 12 API. Amongst the likes of Rise of the Tomb Raider, Gears of War Ultimate and Hitman, they also added Star Wars Battlefront, which is mentioned as the first DirectX 12 enabled game on Xbox One.
Oddly enough, Star Wars Battlefront which is arguably one of the best looking titles on Xbox One ran at a 720p resolution, well below the capabilities of other systems even with the DirectX 12 API in play. This flies in the face of some reports that the DirectX 12 API could assist in pushing the resolution gap closed between console platforms. However this could also mean that the game wasn’t optimized to the maximum potential.
This ties in with comments by Phil Spencer that there wouldn’t be a massive improvement between DX11 and DX12, however that games at the end of this generation would resemble the difference between Perfect Dark Zero and Halo 4 on Xbox 360, which is quite a massive improvement.
With Quantum Break and Gears of War 4 all running at 1080p, which both will be using the new API, perhaps it was only an optimization issue. We’ll just have to wait and see on how much DirectX 12 will really improve the Xbox One, if ever.