Most cards are losing stability ( or generate single artifacts ) at 2100+. Max boost can be 2025MHz but for most of the time clock will be lower. I guess if you make it run at stable 2100MHz then it will be good result. Max for my card was about 2080MHz but it's Founders Ed.
Ya it seems that even on water they lose stability pretty close the same speeds as they do on air. The most I can get out of mine is 2139 MHz. When it gets to around 48-52c which is the hottest it gets, it can drop to 2126 MHz but won't ever go lower than that and stays at 2139 MHz for a good chunk of time.
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.