You can't just assume you're going to get to 850mhz fully stable - in any case you're looking at 1.1v easy. Anything over 725 is good and anything over 800 is great, but not all cards are great. And keep in mind that with this generation of cards, as you increase heat by upping the voltage you really
increase the power consumption, so additional gains beyond a certain point are somewhat muted by those downsides.
I stopped where I am (750 @ 1.037) which is well above reference both in mhz and volts while outshining the original gtx 560 Ti. Performance is top notch, especially with two
No problems at all with bf3 or anything else for that matter.