You shouldn't really need dual rates with the Avistar, though some expo to soften the sticks around the middle might be a good idea. That will help prevent overcontrolling while still having full throw available when you want/need it. Expo tends to be a personal preference. Play with the setting until you get the "feel" you like.
The only improvement I can recommend for the Avistar is stiffer main gear. They tend to spread with even the softest of landings. While they detach quickly and easily to bend back into shape, doing so after every few landings gets old quickly.