I posted again and again and again pcgameshardware results where they test aftermarket cards and 980TI is faster.Its 5-10% faster in every game and its running only at 1350Mhz.GTX1070 running there at 1900Mhz.Overclock both and it's either a tie or the 1070 comes out on top.
Give the 980 TI a healthy 25% overclock performance boost (which was the upper end of a typical 980 TI overclock) and give the 1070 a 12% overclock performance boost (which is fairly conservative) and the 1070 still comes out on top. The 1070 doesn't consume that much more power when overclocked, while the 980 TI has a more noticeable increase in power. Also as Sweeper said 8gb vram. It's definitely not a valid upgrade path for 980 TI owners, but there really is no decision between which card would be better to own when given the choice.
Latest two games tested and in both GTX980TI is faster.
RE7 980TI is 10% faster and 1070 vs 1080 gap is 30%
For honor 980TI is 5% faster and 1070 vs 1080 gap is 26%