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Nihm batteries will lose charge over time if unused or not recharged, so it's probably best to charge them fully if you intend to leave them stored for longer than a month. However, a Nihm will recover from a deep discharge if it's not allowed to stay "dead" for too long. I've recovered a 9.6V 3300 mah Nihm that I mistakenly left connected to my G&P M16 that has an ASCU fitted, despite the warning to disconnect it, for about two weeks. It ran down to 2.1V, but a slow charge brought it back up to full capacity and it doesn't seem to have suffered any harm.
Lipos should hold charge for far longer if left unconnected or not linked to a mosfet. I have a small 7.4V Lipo in my CYMA Dessie AEP and that holds charge for months without any serious loss of charge, even though it's left connected. But don't let a Lipo drop below 3V per cell for more than an hour or two, otherwise you are liable to find it will not recover.
A higher voltage battery will probably give you a small increase in rpm, or try switching to a 7.4V Lipo instead, as that has about the same effect as a 9.6V Nihm for a minimum outlay. Otherwise you would really need to invest in a set of high speed gears and a high speed motor to get a faster rate of fire, but that is bigger bucks and a gearbox strip and rebuild.