I recently purchasing a MSI HD 7850 OC and have been experimenting with overclocking the card. First I used CCC, then Asus GPU Tweak and now I settled on Sapphire Trixx. I was not able to use MSI Afterburner due to system instability when starting it up.
I am able to overclock the core past 1200MHz with a little voltage increase, however anything past ~ 1100MHz at 1.14V gives me decreased performance. The max power slider is set to +20%. I've tried both Furmark and Crysis Warhead benchmarks and the results are consistent. The only thing that makes sense is that the power draw of the card is limited, either by hardware or software.
I realize that the PCIe and 6-pin are technically not designed to provide more than 150W, which is supposedly what the card draws. It's too bad the card only came with one 6-pin power connector. There's no point in overclocking the reference HD 7850 further than the CCC limits because it's power-limited. If there is any way around it, I haven't found it yet.