What if you held the throttle back on the 9 cell pack to apply the same amount of power(same RPM) as the 8 cell pack? Amps x Volts = watts.
OK, using your analogy then for the servos to be the same you would have to move your radio stick slower to make sure they drew the same amount of current as the 4.8 volt pack.
The servos Will draw more current with the 6 volt pack