That's the first logical response I have seen. Now my guess is that like going from DDR3 to DDR4, the latency goes up, and if bandwidth can't overcome that, well that is why good DDR3 2133 is faster than normal DDR4 2133. So my guess is that gddr5x runs faster but has higher latency than gddr5, but then it wouldn't make sense for Nvidia to do that as latency is very important as well as bandwidth. My guess is that windows or drivers are crippling the 1080 right now, that should change.
1080 is graphics card made for playing games. Usually latency is not such big issue there, because typical workload is working on tons of textures and running same shader code on lots and lots of pixels.
And there is always something else to shade while waiting for that data.
So for games GDDR5X is great.