I had a pair of over clocked GTX 680's, which were still very capable as a pair IF the game didn't want to exceed 2gb, and IF SLI scaled well.
For me, Fallout 4 played pretty well at mostly medium settings and SLI was a huge help. I did test 1x 680 vs. 2x 680 and, in the same scene / area I'd see one GPU doing 35-40fps and, once the second was enabled, both sitting at a vSync-limited 60fps quite happily at my settings. An example of good scaling.
However, in certain other titles without SLI support, namely ARK Survival Evolved and X Rebirth, the former was basically unplayable (for me) at any settings, and the latter needed things turned right down for smooth gameplay.
With the 1070 I've cranked ALL settings to max in FO4, plus I have texture/mesh mods and an ENB and the game plays flawlessly at 60fps - bar certain known problem areas. ARK, still woefully unoptimised in my view, runs at a comfortable 50-60fps for the most part, with the odd dip into the 40's - a MASSIVE improvement plus I'd not realised quite how pretty this game could be lol. X Rebirth is now smooth and looks far prettier too, as are various other titles I've tried....didn''t help much in Minecraft...odd that
Re: 1070 vs 1080 - I never personally expected the 1070 to come close to a 1080 - a 20% core cut is too much, meaning the 1070 would need to be (in rough terms) running at a 25% higher clock that the 1080 it's matched against to equal it....more realistically likely more than 25% when ram speed is factored in.
At the end of the day, I made my jump and am enjoying this hardware now. My fps is better now, my visual fidelity is better now....you get my meaning. If I'd gotten a 980Ti I'd likely feel exactly the same.
Scoob.