7850=1024 SP's @860 mhz
7870=1280 SP's @1000mhz
Seems to me you've got to OC the 7850 at LEAST from 860>1100 to match the stock 7870. And I just read an review where I think they maxed their 7850 at 1140 mhz.
But I dont pretend to be an overclocking expert. If you're one of those nuts who's all "if I up the voltage just another .0001 I think I can push it to 1347 mhz with only blue screens every week or so!" types yeah you can probably beat the stock 7870.
There isn't a massive amount in it between a stock 7850 and a 7870
http://www.anandtech.com/bench/Product/549?vs=548, say roughly 20% on the whole.
The 7850's can OC just as high as the 7870's, the only performance difference between the cards is that the 7850 has 20% fewer shaders. Like with the 6850 vs 6870 and 7950 vs 7970, shader counts alone do not majorly affect performance. Clock speeds, ROP's, Memory bandwidth etc. mean that a few missing shaders make little real difference.
Many reviews show a 7850 overclocked to 1050MHz equalling a stock 7870 @ 1000MHz. That mean clock for clock a 7870 is only very slightly faster.
Stock speeds
Stock 7870 = 1000/4800
Stock 7850 = 860/4800
difference = ~20%
before GPU Tweak allowed unlocking average overclocks
OC 7870 = ~1250/5600
OC 7850 = ~1050/5600
difference = ~25%
with GPU Tweak unlocked overclocks
OC 7870 = ~1250/5600
OC 7850 = ~1250/5600
difference = 6-8%
The 7850 with a good third party cooler like the Twin Frozr 3 can usually hit 1200MHz with a voltage tweak (mine is fine for 1200/6000 at 1.185v) which is taking it up to stock 7950 levels.