The Space Scooter's gearbox is 3:1 AFAIK, so unless the stock box is broken, there really isn't any benefit to changing it. Doesn't hurt to have a spare around, though... Stuff happens

You're exactly right about the relationship of control horn length to servo arm length. The farther away from the center of the servo, and the closer to the control surface, the more throw you get. However, elevator throw doesn't make the plane climb faster. Elevator only controls pitch, power and aerodynamics determine climb rate. Hence the very popular saying, "The elevator is not the magic up stick!" If you pitch the nose up without enough power, the plane stalls and goes down. If you pitch the nose up too quickly, by having too much elevator throw, the plane will also stall even if you have adequate power.