The 1070 and 1080 show good gains. 1080 is dx12 king at the moment and the 1070 is showing good gains over 980ti in something like hitman and is rivaling the Fury X (like it should as it is a ~980ti performing card).
They certainly don't have a "considerable" advantage against the 1000 series.
http://www.guru3d.com/articles_pages/nvidia_geforce_gtx_1070_review,14.html
http://www.guru3d.com/articles_pages/nvidia_geforce_gtx_1070_review,15.html
Errrr..... Thats because Nvidia have released their latest generation of cards and AMDs are yet to land, AMD still have an advantage. Especially against price.
You're impressed by the 1070 in DX12? When it's being matched by a 390x.... Which if it turns out true, the 480 (A £200 odd card) will match?