The capacity of a battery (mAh) makes a difference to it's discharge rate too, a 2600mAh 15C battery will kick out a lot more amps than a 800mAh 25C battery for example. more amps = better trigger response and faster rate of fire.
I'd always recommend going for the biggest battery you can fit. You should be able to get a WE airsoft 1450mAh 7.4v LiPo into that stock no problem, they're what I use in my guns now and I've had no cause to complain at all.