Like Gam3r01 said, it is impossible to say. Some chips overclock better than others. I had an AMD Radeon HD 7850 a while ago, which is essentially the same card as they use the same GPU die. I know in general what some other people got and what I got on mine, but that doesn't mean you will get the same results.
The highest I saw anyone ever hit with one is about 1400 MHz with an extreme increase in voltage. The most I ever got was 1180 MHz, which would give you roughly a 10% increase in performance if you managed to hit that. At the same time, my brother also had one and his couldn't go past 1050 MHz without being unstable. So you may not be able to overclock it at all without it becoming unstable.