99.9% of the time you can get just as good a clock, sometimes higher as the companies still have to keep it stable on what is basically a stock card. There are things the home user can do, like increasing voltages to stabilize a higher clock that most factory overclocked cards bother with.
However there are cards out there (and this is where doing some digging and reading some decent reviews can pay dividends), that have non stock parts and settings that will help you get even higher clocks. Asus i think it is make a 3850 that has a button on it that increases the voltages to assist when overclocking. Some of the Factory overclocked cards are said to have the best of the best chips on them, both GPU and Memory wise. While this is true for some of them the vast majority of them are just stock cards ramped up to a level easily attainable by the end user, just to get more money out of us.
Mactronix