Overclocking can improve the performance significantly, if done correctly.
Unless you have one of the paid versions, you cannot change the benchmark settings.
In the thread I linked, there's somebody with a 780 (non-TI) who could probably give some tips on how to improve your score if you are concerned about it. Personally, though, if the computer plays games well enough that you are happy with it, I'd just leave it alone.
Casey