AK47frizzle

Another good way to up your ROF is to do a load of little tweaks to increase your air efficiency, push the max FPS and then drop the spring power (most gearboxes lose 20% of their power due to not being air efficient, fix that and you can put a much weaker spring in)

AND, finally, if you just want a standard 'this' answer, if you can only do gears OR battery, go gears.

if your gun isn't up to it you will be rebuilding the gearbox anyway in 6months or less if teeth start to strip/over run.

Ther are now LiFe batteries @ 9.9v which may be a more gentle trade off for your gun