That would make sense, but the benchmarks I've seen show it performing quite a bit better than that, at least at 1080p. This is probably attributable to higher stock clocks on the 960, and the fact that practically every launch 960 is overclocked on top of that. The 980 seems to be ~70% faster on average.Benchmarks:
I believe the GTX960 has exactly HALF the processing units (and memory bandwidth) of the GTX980 thus it should perform almost exactly HALF as well in games.
So just look at GTX980 benchmarks and cut the FPS in half (assuming no CPU bottleneck).