I have a 9.6V 1800 mAh battery in my C8A2. That, along with a Prometheus MS110SP spring gets me about 360 fps and excellent rof. Battery voltage won't affect your power in any way. It will only affect rof and capacity.
And if your M15 is stock, you should be good to get at least 2000 rounds out of that 2000 mAh battery. A stiffer spring will decrease that slightly.
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