AFAIK Xbox 360 and now one effectively ran a low level version of DX and a almost normal versions of windows DX(of the time). One for serious developers to have the option to get every piece of performance possible and one for dev costs, largely for indie games, to be able to target one API that is also simpler. As the majority of indie games are less performance intensive the ability to have low level access, control every aspect of memory(I'm not sure if this would be abstracted out on the higher level API on the console or not) and less potential problems.
So DX12 being put on the console really just potentially helps more likely indie devs who can use one api across windows/xbox and potentially choose a few more features that enable performance if they want to. But due to costs, potential for screw ups with more control it's unlikely to offer much of anything as the guys using the higher level api on the Xbox One probably aren't looking for the new lower level features of DX12.
In terms of DX12 low level access vs the specifically designed low level API currently on the Xbox one, I would think best case scenario is there is no actual difference IE it would be no better but offer a different route to the console, more likely is it will still abstract things significantly further than the existing low level API.
PS3 was the same and I presume with PS4, in that they had options for different devs/markets/requirements. They had low level api to get the maximum possible and a higher level "easier" API to use for those that didn't require low level access. AMD has said the same for Mantle, if you want the power and control it's great, but if you need neither, there isn't a huge amount of reason to implement your game in Mantle(or any low level API).
