1080 is in a bit of a strange position - 8GB of VRAM is fairly decent and putting GDDR5X on it is pushing the boat out a bit, but then you've got a part that is far from pushing the boundaries of what is possible on 16nm, using largely penny pinching electronics on the PCB rather than what you'd normally find on a higher end bracket card and performance that is closer to what you'd get from a refresh than a generation leap (which is usually where you find the mid-range next generation cards).
I think the 1080 finds itself in difficulty on here because many people have very highly clocked 980Tis and rightly or wrongly comparing it with them.
Stock for stock it is a good 30% faster, sometime 40% in DX12 compared to the 980Ti which is actually not bad considering it is the mid range part.
Unfortunately, so far it doesn't look like it overclocks as well, meaning once both are highly clocked that advantage drops to more like ~20%.
Obviously part of the reason for this is Nvidia going for high clocks so to get the same percentage overclock compared to a 980Ti you need about 2.4ghz+.
For people that don't do overclocking it is a decent enough jump forward. For those lucky enough to have a 980Ti that does 1500mhz fully stable in everything, it obviously isn't.
The same goes for the 1070, it is slightly faster than the 980 ti stock for stock, especially in dx12 but then slightly slower once both are given a decent overclock, simply because the 980Ti clocks better.
When you look at it. We were spoilt with Maxwell overclocking. I mean the 980ti reference boost was 1075mhz and most can do what, 1450mhz with some doing 1500mhz or more? I mean that is a ~40% overclock, easily done on air, which I doubt we will see again for a while.