Any HD5850 should really be able to reach that factory OC so I doubt they are binned. As for the warranty i doubt they differ at all. The card is specifically advertised on it's overclocking abilities.
So basically it's price that should decide things.
Sure, but it makes sense that they are possibly higher binned, just as the case is when it comes to CPUs and RAM. I think the point was that OC'ing would maybe have no effect on the warranty because the card comes OC'd. Regardless, price still dictates that I buy the OC'd card.