Because we already have proof that Windows 10 gaming is much better driver side than Windows 8.1 you follow anything AMD you should know this by now..
DX12 a low level API just like Mantle shows very strong gains when you have high bandwidth like shown in 290x so you would also think the gains here will again be very interesting.
FYI Check Guru3D
What Nvidia do with DX12 is anyone guess but so far we can look at Draw Calls, again shown on Guru3D the better the drawcalls the better the Driver is performing.. So with all that said so far AMD seem to be going in the right direction with DX12.
Am excited about dx12 are you?
See here
https://pbs.twimg.com/media/CHzD-h_UEAEXSJy.png