Battery voltage has nothing to do with mechboxes. Whether its a ver.2 or a ver.6 it doesn't effect anything to run a larger battery like a LiPo.
The LiPo issues are that the ROF goes way up so if the spring isn't powerful enough to return the piston fast enough it can rip the ass end off your piston. The higher amperage causes a lot more arcing across the trigger contacts which can wear them down a lot faster so a MOSFET is usually a good idea. How fast your gun shoots has nothing to do with how strong your mechbox is. The only issue is a mechbox will usually crack after so many shots, a higher ROF will just get you there faster.
One exception is an absolutely insane ROF, like say 60+ rounds/second (something I have never heard achieved here on ASC) where the kinetic energy from one shot has not totally dissipated before the piston hits again so it can reach a higher impact strength through rapid succession.