A GTX970 is around 55% to 60% faster than a GTX960:
https://www.techpowerup.com/reviews/ASUS/GTX_950/23.html
So that is around the upgrade if a GTX1060 is around that level.
A stock GTX980TI is around 36% faster than a GTX970 in that link.
So if Nvidia does increments of 30% performance boosts at each level.
That would put the GTX1070 at stock GTX980TI level and the GTX1080 at the purported 30% faster level.
That is around what we saw with the GTX680 and GTX670 against the GTX580 and GTX570.
The GTX660TI was nearly 50% faster than the GTX560TI.
The number seem consistent with Kepler.