You are correct. The servos will move faster and generate more torque. The downside is the battery will discharge faster.

If you are going to a 6volt battery I suggest the following:

1. While MOST receivers and servos can handle 6 volts with no problems, check to make sure the brand(s) models of receiver and servos will handle 6 volts.

2. Run a larger capacity battery than you would normally use. For instance, if you use a 4.8 volt, 1,000 mah battery, I would use an 1,800 mah battery when you go to 6 volt. The extra capacity SHOULD insure that you have a similar flying time before having to recharge.

3. If you are going to use digital servos with a 6 volt battery, I would up the capacity of the battery even more as digitals do drain a battery more quickly than analog servos.