The general consensus is the 980ti was quite a bit more powerful than expected. As far as I remember, the performance gap between the 980ti and 980 was greater than the gap between the 780ti and the 780, yet the 980 was not that much more powerful than the 780ti—at least at launch with early drivers and older games. As everyone knows, the 980's area of prowess was its efficiency. A 980 at 1150/7000 was only marginally more powerful than a 780ti, and in older games the 780ti was more powerful. It wasn't until we started seeing 980's at 1400/7200Mhz and beyond that the gap widened. Drivers helped as well, as did the games being developed around the Maxwell architecture.
So, in my opinion, it stands to reason that the 1080 will initially only slightly surpass a good 980ti. We'll see the improvements in the way of drivers, greater efficiencies (1080 SLI with a 750W PSU for $900, anyone?), and an updated architecture that developers can utilise to a greater degree.
Realistically, if the 1080 was considerably more powerful than the 980ti, that means it'll be as powerful as 980 SLI, which doesn't correspond with past generations. The 980 wasn't as powerful as SLI 780's. You had to overclock a 980ti to reach that kind of level. Similarly, the 780ti wasn't as powerful as 680 SLI. While the die shrink should theoretically net higher performance, again the 980ti was far more powerful than expected. It slayed.