I've never messed with the 1070s, but they're the same core with less heat as the 1080s so I can't imagine they'd be much different. I've owned 5 1080s and not a single one of them wouldn't overclock to at least 2100 on air with just setting the offsets. They'd all do at least 2150 with mild corrections to the frequency / voltage curve. 3 of them would do 2200, but not run as well as 2150 would. Now on water, both of them (the ones I kept) will run 2200 with normal ambients and with lowered ambients have hit runs in SLI @ 2252.
The frequency / voltage curve is the key....that and temps, temps, temps. The lower, the better.
To err, on the side of caution, maybe around 2100...if you get more, smile big.