DO NOT use the smaller, Futaba-specific servo saver for a touring car. That servo-saver is designed for a 1/12th scale car, and the spring in it is not strong enough for a touring car (at least on carpet or other high-traction surfaces). It will work, but you will not get all of the torque your servo can provide.
The saver you need is the Kimbrough 201, which is the "Mid-Size w/ holes".
This one is strong enough for a sedan, but smaller than their "Large" servo saver, which would not fit in the RDX.