First up Overclock3D received slides from ASUS that show off the performance level of the GTX 1070 Ti in regular and OC mode. As is typical for this kind of slides, whoever compiled them used the traditional "marketing scale" to maximize performance gains:
Next there are also performance slides from Gainward, you can view those pictures at VideoCardz.
According to the chart from Gainward’s GameSoul GTX 1070 Ti product page, overclocked GTX 1070 Ti (running at clock speeds ~11% higher than stock) will still be slower than GeForce GTX 1080, at least in 3DMark Fire Strike Extreme. If true, then this is very interesting because one would think that GTX 1070 Ti with 188 MHz higher clock would easily outperform reference GTX 1080.