I have successfully used airodynamic balances on both rudder and elevator to allow the lower powered servos (futab148's) to move very large surfaces on 1/4 scale models. Worked well with adequate throw and no servo stalling. In the process of my experiments, I did find that it is best to keep that area forward of the hinge line to not exceed 10% of the surface or I got hunting or oscillations around neautral. You can make these surfaces do dual purpose as; properly weighted, they also provide static balance which can be very helpful on elevators and ailerons.