I think you mean at stock, and it is... OC vs OC the performance gap is much smaller.
The major difference there is both are from Nv and designed/clocked so there is a difference between a top end refresh card and a mid-high end card.
3% overall while consuming more power, running louder, and putting out more heat isn't selling me for $50 cheaper. (Opinion)
Even a stock 6970 was fairly close to a stock 580, $370 vs $500 - AMD still wasn't able to keep market share.
But then we're seeing cards like the 680 at $500, the 7970 at $550 and now a halo card at $1000. I guess the days of reasonable prices are gone, and the move is backed up by the end users it seems.
I just don't see the point of buying a inferior card for $50 less when you're already spending $450, the savings is pretty much pointless by that time isn't it?