I was searching for information on the IFI system over the summer, and I found this link:
http://qin.laya.com/first/joystick.html
That code is in PBASIC (as used on the previous generation of controllers), but you probably should be able to rewrite it. (Chalk it up as a learning experience.
) Basically, that link teaches you how to set up the motor to speed up on an exponential curve, instead of linear. Slow when you start, but still full power when you push it all the way up.
Hope it helps!