The watts (amps x volts) is the power limiting factor. How efficiently the power is converted into thrust and how much load is created by weight and drag are the two largest factors that determine performance. The lighter the plane, the more efficient the motor, prop and speed controller, the faster and longer the flight. The number of cells should depend on the maximum amperage sustainable by the motor windings without over-heating the motor or the batteries. A direct drive is more efficient than a gear box. Choosing the right size and pitch prop for a given motor is the key to achieving optimum performance from a given motor battery combination. The weight should always be minimized and the drag as low as possible to achieve the fastest airspeed. The power can be increased by adding cells, but this also adds weight. Choosing the right voltage motor (different number of amp turns and wire guage) allows the power to be varied. Since the batteries have a fixed amount of stored energy, i.e. 1.5 Amphours (1500 maH) at 8.4 Volts = 12.6 watts for 1 hour,
this means that your flight time will be shorter the more watts you use to fly the plane. For example, if you are drawing 15 amps in flight, the batteries will last for 60 min x 1.5 AH/15A = 6 minutes. Also, the batteries will get quite hot at that 10X discharge rate. Adding cells will increase the volts and amps and therefore the watts, but could result in overcurrent conditions for both the motor and the batteries. The power is battery limited. Using larger cells 3300maH will reduce the heat and battery overcurrent heating. As always, everything is related to everything else. If a given motor is twice as efficient at converting the input power to thrust, the plane can fly twice as fast, or twice as long with the same batteries.