I have used boost tabs on both rudder and elevator on large models. This was before the more powerful servos were readily available. I found them very useful. I also found that the area of the boost tab should NOT exceed 10% of the area of the moveable surface else the boost tab may overpower the servo in some instances. All my experiences with boost tabs has been very rewarding and let weak servos (compared to modern ones) do a very effective job of giving adequate authority to large control surfaces.