i dont like comapring the oced lower card to stock higher card. this is just stupid. the higher card can be oced to and it will be in front again. comparing should always be done at stock vs stock.
Of course the higher, more expensive card will be more powerful.
But at the price point of the lower card, you can match the stock performance of the higher card at no additional costs.
So for example, you can overclock, lets say, a GTX 560 Ti for $250 plus shipping, and if you get lucky, be able to match a GTX 570~580 for only $250 instead of the $350 or $500.
It's not saying that the lower card can be more powerful always, just at that price point.