I can confirm that the 1070 is ample for 2.5K gaming. I'm currently running one with a 4K monitor, and while it doesn't play 4K very well (WoW runs ok at 4K, but not much else), pretty much anything I throw at it in 2.5K rez works great. For example, Doom (latest release), Space Engineers, Witcher 3, all with settings at max or near max. 50-60 fps without issues.
I'm running the Asus ROG STRIX Gaming, with a very slight overclock and fan adjustment.