I realize this is an old thread, but I just got a 7850 and wanted to post my OC results.
I have the Sapphire 7850 OC, and so far I have it stable at 1160mhz core and 1350mhz memory. I had to increase volts to 1.225V to get it stable though.
This thing is an overclocking monster! Consider that reference is 860/1200!
Edit: also it stays cool at 69 degrees, although It once got up to 72 after a few hours of looping unigine heaven 3.0