it varies from card to card even the same card from the same vendor can vary in how they oc , certain manufacturers will "cherry pick" cores and fit high quality ram to their boards as well as high quality capacitors etc and they up the voltage in the cards bios and fit better cooling , these are usually the factory overclocked models which you pay a premium for , overclocking these any further is usually difficult , sometimes if your very lucky you may end up with a cheap "base" model gpu that clocks very well but as they usually have cheaper lower quality components / ram they are also hard to get a decent oc on without a volt mod
generly speaking unless you can get get a good oc on the cards ram (higher bandwidth) its not worth overclocking its alot of hard work for 2 or 3 fps , people normaly oc for higher benchmark scores , ingame the differance is usually very small