If the 1060's were equal, with the only difference being the amount of VRAM, I could potentially see the argument for "overkill".
But that's not the case. The 3GB 1060 is a 'crippled' 1060 - realistically, it should've been called something else entirely.
It's likely going to end up somewhere between a 1050 (based on rumored spec) and a 1060. Something like a 1050 TI Boost or similar would likely have been a smarter/clearer naming convention for the card.
Either way, even the 3GB variant is a huge performance boost over a 960 which, as I mentioned to begin with, was a pretty poor investment (read "bang for buck") even when new....... kinda comparable to a 3GB 1060 today! (Not in a sense of raw performance, but in the poor "bang for buck" angle.