[mcguyver]

Capacitors don't generate any heat at all. I'm skeptical about their use for increasing ROF as you would need a large capacitance to bring the lagging current from motor inductance to unity. A smaller capacitance will just increase net power factor, but will still have enough lagging current (coupled with a small battery) to really offer small improvement and not worth the time and expense, IMHO. And remember that capacitors in a DC system will store energy indefinately unless drained, which will be a manual procedure as capacitors in AC systems get drained during current reversal. Stored energy like that can be a potential danger, not to you per se, but if accidentally shorted or reversed polarity, may explode, destroying your gun.

The idea of using different tap points on a battery will work, but you'll need a 3rd contact point on you switch to make it work. You would have 1 common output to motor, 1 tap for 8.4v and 1 tap for 9.6v. Too much work and mods to make it worthwhile IMHO. Look at an automotive relay or a 2 pole switch as this is the type of switching you'll need.