You basically hit the nail on the head. Expo makes the servo respond in a non-linear manner to the stick input. This could be used to make the surface move less than the stick input to soften sensitivity or more than the input to increase sensitivity. It is generally only used to soften control sensetivity around center though. It can be really helpful on some very responsive airplanes.

On your Spektrum radio, a positive exponential value will soften the controls while a negative exponetial will make it more sensitive. This is the exact opposite of Futaba and other radios. With them, negative expo softens the controls near center.

On your T-28 Iwould try it with no expo. The recomended control throws fly the plane well and it is a fairly gentle flyer.