you can get 980 (4gb) performance from the 10x0 series for $200?
I do see the 1060 3gb are around $200, maybe a bit less after rebate. but the 6gb are $250+
Maybe it's possible to extrapolate from different comparisons.
This is all I can say, from my recent personal experience and keeping-up reading chores.
I have a single GTX 1070. I have been wondering whether or not to get a second one. But that card with a 1440 gaming monitor and its 8GB of VRAM, 144Hz refresh rate or certainly 60 is no problem.
Further, the card falls behind an i7-4790K and 2x SLI GTX-980 at ~17,000 on Firestrike benchmark by about 800 points or 16,200, both systems overclocked. That's with a fairly easy and safe overclock that proves out in Valley or games I play at a solid 2,038 Mhz core, 4,440 Mhz memory. The temperature in any of those demand or stress situations can go as high as 65C in Windows 10, and about 62C in Windows 7 -- or that's how I remember. I'll have to reboot to recheck.
Maybe I'll take longer to think about it, or maybe I'll move up the schedule for getting a ZOWIE monitor so I can make a solid decision about SLI.