Kevin--I'm no expert, but have learned that more powerful servos draw more current, especially digital servos. Why not go to a 6V battery. A nickle metal hydride at 1200mah does not cost very much and will give your servos quicker repsonse and more torque--even standard servos. Just make sure you charge it sufficiently. If you are using a 50mah wall wart charger (like what came with your radio) you need to charge for at least 24 hours. 1200/50=24.

Good luck.

Greg