How big is that plane?

Running a 6v battery will actually cause a slightly higher current drain, not a lower one. So when you move from 4.8v to 6v, you need to slightly increase not decrease your battery capacity. The advantage with 6v isn't in the battery lasting longer (it's shorter), it's that you get more torque and speed from the servos, and you get a small bit of saftey, since if you drop a cell, you wind up at 4.8v rather than 3.6v