ORIGINAL: sandal
Exponential will not work in this case because the half throttle point will remain unchanged. Expo only works from half stick and towards each end. Since we're talking about moving 1/3 stick to 1/2, I prefer to use a P.MIX curve instead of expo. OTOH expo works well if you want to change the curve near the end points.
While this is true for Ail, Elev & Rud channels, the throttle expo on the 9c (can't speak for other radios) is spread from low to high. I use throttle expo on all of my planes to get the rpms where I want them on the stick. I have used the 5 point mix as well, but I have found the expo method to work fine for my needs and is less hassle.
Mike