Average 10% difference compared to 2x 6870s depending on the benchmark and game.http://www.anandtech.com/bench/Product/302?vs=301
Down side to xfiring 6850 with a 6870 is the latter will disable some of its features to match the 6850 to be compatible.http://www.hwcompare.com/5999/radeon-hd-6850-vs-radeon-hd-6870/
Overclocking the 6850 might help a little more, but you'll still be missing out on some Stream Processors and Texture Address Units.
You could take the risk and flash the 6850's bios with the 70's. But if 6850 is your only choice i wouldn’t worry about it, the two cards together will still kick ass.