potatos produce direct current... due to the lack of alternating voltage, overclocking is unlikely.
Now, if all you want to do is produce alternating current from a potato battery (or any dc source), your clockrate is limited by how fast you can switch the electrodes back and forth. Of course, a true overclocker does everything manually. By my estimate, with practice, a 3-4 Hz increase could be realistic over short periods of time and increased stability should come with experience. I would suggest a large fan, as well as some water cooling for yourself, however, I doubt 24/7 stability is possible, regardless of the cooling method used. On the plus side, a 3 Hz increase over 0 Hz is technically an increase of infinite percent. The limit of (3/X) as x approaches zero from the right equals infinity. Just don't approach from the left. In that case you would have a decrease of negative infinite percent, and if left that way for too long, risks contributing to an earlier heat death of the universe.
note: its been over a decade since calculus, math may be a little rusty.