one thing to consider when using 6 volt packs is that you surely get more torque from the servo, but remember this, servos and gear trains are all built with 4.8 volts as the primary power........we then kick up the voltage to get more power from the servo with a 6 volt pack, and are getting it, but now we are asking the gear train to work much harder as well...............it's best to use servos that are meeting the torque needed to handle the plane in question. think about it, you spend how much on a nice scale plane like the GeeBee, power it with a superb engine like the OS1.20surpassIII w/pump, and do all the expert work it took to get it airworthy, and then take standard servos that most guys will only use for throttle control and try to make them something they are not by using a 6 volt pack?
i'd use no less than S9001's for that plane.......they cost a whole 15 bucks more than 3004's, have better mechanics not to mention superb centering, and will last you longer as well.

it's your plane, but for the investment you have into this, is it really worth saving 50-90 bucks by using servos that are best used in a trainer?