you may want to look into a mosfet switch. it will increase your motors durability and ROF. I would worry about a high rate of fire unless you don't mind breaking your gun. I would stick with an 8.4v, upgrades can be done more than one way. If you use small batteries, adding a couple capacitors will buffer your power source and give your motor a more consistent power.
read more here,
http://www.airsoftmechanics.com/articles.php?aid=11 and here,
http://www.airsoftmechanics.com/guides.php?aid=14