DS821's are excellent servos, and are suitable for everything except maybe rudder, where more power is good. Run them on 6 volts for more speed and power. I know of many fliers that use them successfully, even in some of the upper classes. they are very precise, plenty of power for ailerons and elevators, and fast enough on 6 volts. With the money you save, buy and burn more fuel to practice.

Jon