The amount any given card will overclock varies, even if the cards are "identical." We sometimes refer to that as the silicon lottery.
Unless you need the extra performance though, the only things overclocking will do is consume more power, produce more heat, possibly introduce instability, and possibly (especially if you increase voltage) shorten component life or even outright kill your card. If you want to overclock just for the sake of having done it, do not increase voltage, and you should at least not be likely to do any damage.