Well the nominal voltage is 6v but they run quite happily on 7.4v(2s) lipo
I belive Dave W has successfully run F.I.N on 11.1v (3s) but with a fully charged 3 cell that seems a bit risky.
I would only overvolt the pololu model ones (2 cell) the cheap ebay ones are probably terrible quality and wouldn't like overvolting.