GTX1080 is 21% faster on average than a comparable GTX1070.
I'm still rocking a GTX680, and the GTX1080 is about 2.5X the performance though gets closer to 3X (with good CPU too) in some games.
I play several games at 2560x1440, maxed or near-max, 60FPS. Though most games are 1920x1080.
The GTX1070 is still a great card for 2560x1440, and the savings could be used elsewhere such as better monitor or whatever.
THOUGH DO NOT OVERPAY IF PRICES HIGH. MAY NEED TO WAIT SEVERAL WEEKS.
*Many people don't tweak games well. Doing so goes a LONG way to a better experience. Even with a GTX1080 you don't want to just crank Ultra settings and play with whatever frame rate that offers (unless you don't drop below your target frame rate).
You may want to drop AA or other settings a bit for demanding games, or use Adaptive VSYNC (which toggles VSYNC ON and OFF at the refresh rate/FPS target).