I've only recently got a 1070 in addition to the 980Ti and this is something I've been thinking about. The 1070 FE I have does boost a lot without manual overclocking, so I'm not sure how much more headroom you can get out of that. Take my TineSpy example, it was boosting to around 1900, and it doesn't sound like people are getting that much more above that before hitting a wall.
On the other hand, the 980Ti has a lot more cores and in benchmarking it still seems competitive, if you can get the clocks up. Hence my looking at water cooling recently. Mine is a reference one at 1076 and is already at its thermal limit. I hope once I remove that, it can really start pushing out bigger numbers, but of course after market cooling will add a lot to the cost as I'm finding... I could buy another 1070 for the cost of a decent water setup for the 980Ti.