Expo on rotary servos allows the actual surface throw to be closer to linear, but you need to use about 30% to achieve this (positive expo) If you want to soften the neutral then you need to use more than this. As far as the aileron response is concerned, reducing the servo movement cuts down the number of `steps' (1024/60%) and might result in a slightly less `smooth' servo response, but I doubt you will notice anything when you are flying, if it is a concern, then just don't use full stick movement when rolling, it is supposed to be `proportional' radio after all.
Evan, WB#12.