I would look to the Titan X and judge the 1080 Ti as being essentially the same.
Comparing spec for spec, the GPU is identical but for an extremely marginal overclock. In fact the only thing that seems to have changed is the memory. it has 1GB less vram (which implies that they cut it down by disabling a memory controller, like they did with GM204). It also has a smaller bus, but at a higher speed to give almost identical memory bandwidth.
It might be that the 1080 Ti is higher binned than the Titan X which would result GPUboost 3.0 taking it further at "stock" (because if not claiming that this is a better gaming card than the Titan X based only on a 30 MHz overclock would be absurd).
I'm a bit disappointed. On the one hand I thought it might be a further cut down Titan X, but it isn't, but on the other I had hoped for a full 3840 CUDA core GP102, and this clearly isn't it. I guess Nvidia just don't consider Vega to be a threat.
So yeah tl;dr pretend it's a £700 Titan X and you're not going to be too far off.