Both GTX 780 Ti and GTX 980 Ti were launched as response to AMD products. GTX 780 Ti was launch as response to R9 290X, and GTX 980 Ti was launched to preempt R9 Fury X. So unless AMD has something that beats GTX 1080 and gets-close-to/outperforms GTX TITAN XPascal, NVIDIA won't launch GTX 1080 Ti. If AMD's response is slower than TITAN X Pascal, GTX 1080 Ti will have fewer CUDA cores. If it's faster then NVIDIA will max out 3,840 CUDA cores present on the GP102. If it's way faster, then NVIDIA will tap into GP100. So no, GTX 1080 Ti probably won't launch till Vega10 is close, and it's not.
At this rate GP100 won't make a market debut under GeForce brand this generation. It will be shelved for the GeForce 20 series.