If you overclock a 7970 to 1050-1100 6000-6300 then it is about equal under extreme load, where you really need FPS, so about equal for practical purposes with those overclocks.
And considering most can reach 1200 6300 + overclocks on stock cooling with 60%+ fan, thus it is just like 5-10% slower than the GTX 680 when it loses and sometimes faster than an overclocked 680 and mostly close enough for you to not be able to differentiate, that is a sub 5-10% difference, sometimes one wins, sometimes the other
To make this an upgrade, you should be coming from a 7800/580/6900 series or lower, not 7900 series, not even a 7950 considering you could have gone for the 7970 instead if you wanted every bit of performance (instead of the 7950 that is).
But coming from GTX 580/570 or Radeon 6900, it does make sense. But the radeon isn't exactly bad, but would have been perfect at $450-500