Read quite a few reviews on the 280x and they did say that the DD cooler was completely redesigned and worked very well.
Load temps were under 70c, while the 770 reached 75c.
OC results on the DD suggest ~85Mhz extra is all you can get on the core, and an extra 50Mhz on the ram. I think voltage bumps were required. This netted a nice FPS bump which pushed it above a non-OC'd 770 in most games.
Problem with that cheap $250 760, it doesn't have a factory OC, and with only 1 fan, I'm assuming the cooling is pretty average and won't support much of an OC. Still, if you didn't need as much performance, $250 is an awesome deal for a 760.