A 1070 Ti makes a lot more sense as eventually yields are going to improve and there's no good reason to cut out 5 SMs if only 2 are bad.
Do you remember GTX770 vs 760?
760 was cut same as is 1070 vs 1080.
And i dont think it was because yields problem(on 2years mature 28nm)
They just want this HUGE GAP in performance.