660Ti is faster than 580:
http://www.anandtech.com/bench/product/860?vs=767
Also, 660Ti has 2.5 TFlops of compute, 580 ~1.6 TFlops (per Wikipedia).
980Ti at stock Nvidia specs is probably a tier below 1070 at stock Nvidia specs.. The official 980ti is only 1000 mhz base / 1075 mhz boost (geforce.com). Any OC card though for each (i.e. MSI 980Ti, MSI 1070) and they're probably about the same tier..