That certainly is a very unusual way to set up a plane, and nothing I'd recommend to a newby. 100% expo would make the control surfaces unresponsive about +/- 25% either side of center stick. The only control you have would be at the extremes of your stick movement. It seems it would be almost like non-proportional control, as you move the stick nothing happens, move it almost to full stick then all of a sudden the surface responds.

As far as 15 - 20% throws; that doesn't tell much because it's surface travel that's important and it depends on how your linkage is set up. For that to work you'd have to have your linkage set up with a very poor mechanical advantage. Ideally linkages should be set up to give max deflection at 100% servo travel. If you have a computer radio which can drive the servos at 150% then set max deflection at that or as much as you can get. That way you have more precise control at small stick movements and you are multiplying servo torque to the surface. I shoot for a 2:1 mechanical advantage, or as close to it as I can.

Also your comment about not much throw being necessary at high speed is true, however at some point (like take offs and landings) you will need to slow the plane down. Without adequate throw you won't have sufficient control when you really need it.

Some guys use dual rates to give more throw for slow flight, I prefer to use 25-40% (depending on the set up) to have smooth control at high speed and sufficient control slow. Not having to switch rates is one less thing to worry about while flying.

Joe