i do not think it is the torgue of the servo that will cause the spring not to be strong enough to make it return. i think it is more of how the gears are geared down, if they are lubricated, the tightness of their mesh, and if they are all steal. i think the combined factor of these elements will determine if the servo will be too tight to come back. the bottow line is all servos are motors with gears attached. how well the gears are disigned and maintained will give you the smoothness of the servo.