Which make? someone else may be able to help.
I know the MSI ones will go to 1400MHz without even increasing the power rating, and that's i believe +213MHz on the clock
haven't tried it myself, but i know someone who did. Right now i am getting 39MH/s on my 3 750Ti's with the clock at +113MHz
so the clock rate doesn't really need to go very high to achieve 13MH/s a card
Some of them are overclocked by the factory so have less headroom for further overclocking I imagine.
I have a Gainward and a Gigabyte and the Gainward can't be dialed up as high as the Gigabyte. Gigabyte is at 1301mhz (+125 overclock) and the Gainward is at 1317mhz (+89mhz).