The thing you have to be careful with on expo is that if you go much beyond a reasonable amount, say 15%, is that the car will feel dead off center and then the steering comes on when you don't expect it. I once tried a guy's car who had like 35% expo in it without him telling me. I was wondering why the car pushed and then POW it went into the board like something was broke. I did this like 3 times and then asked what was wrong with his car...he then told me he runs an ass load of expo
What I do find works better is to reduce the steering speed if that is available on your radio. It slows the servo down in a linear fashion, which is much more natural.