Yes, you can compare a oced 7850 to stock 6xxx series card.
7800 and 7900 are sureshot overclockers and overclock by a huge margin and scale nearly perfectly with the overclock.
On the other hand a card like 6970 can at most overclock by 5-7% and the real increase in performance with a 7% overclock is usually less than 2% in terms of FPS. Because the 6800 and 6900 architecture doesn't increase FPS adequately even for benchmarking purposes, no matter how high you overclock them which you can't do anyway because they don't overclock, leave alone being sure shot overclockers.
A 7850 is sort of guaranteed to overclocked 25-30% on stock volts, with the stock cooler, you just need to open MSI Afterburner and set the overclock by 20-25% more speed and forget about it. The previous graphic cards were not like that leave alone 6xxx series.
In short, 7850 OC ~ 580 stock > 6970 stock
7850 OC ~ 580 stock > 6970 OC no matter how high the 6970 is oced.
Also, 7850 stock > 10% faster than 6950 stock
But once overclocked, a stock 7850 with the stock cooler will beat the crap out of any 6970 with water cooling and perhaps even with LN2 for that matter because the 6900 series doesn't scale with higher speeds, it just means a higher speed, not higher FPS. This isn't so with the the 7800 series.
And yes, going from 6950 to 7850 doesn't make sense unless it costs less than 5-6k net after selling the old card. But from 6870 OC to 7850 OC is a huge difference, night and day sorts.
And the thing is that AMD was probably giving offers to win market share back, now that they are doing better, they don't need to be so cheap.