Both are a bit "too much" for 1080p.... but no way would I call them "about equal in performance"... at least not "out of the box".
However, the 1070, like all x70s is intentionally gimped" so as not to compete to well with the 1080.
The 980 Ti on the other hand, depending upon which one you pick, can overclock as much as 31% ... the 1070s about 15.1% ... so when both are overclocked, it's much closer
MSI 1070 Gaming X = (100% x 115.1 / 100) = 115.1
Gigabyte 980 Ti G1 Windforce = 85% x (131.4 / 100) = 11.7
As for "bottlenecking", it's probably the most misused term next to "future proof".... It wouldn't even bottleneck a Sandy bridge 2500k from 2011.