GTX980TI got faster memory bandwidth than GTX1080.
GTX1070 uses 8Ghz GDDR5. So bandwidth to processing power the GTX1070 is better on than the GTX1080. In other words, bandwidth is the least of the GTX1070 worries.
What is that faster memory bandwidth?
ps. I think that NV made sure that you can't catch up to the 1080 by simply overclocking the 1070.