I can't say anything about the kammback, I'm to stupid for that, but an underbelly pan would be worth looking into as well. I would say an air-dam instead of underbelly but that would require drilling which you don't want to do.
About the automatic, you can just coast-down by popping it into neutral can't you? I would advise against engine off coasting with an automatic because it causes major damage.
Oh also, get an ultra-gauge, this will be invaluable to improving your driving habbits, and will discourage "road rage" the first time you do it and watch your mpg go from normal to ~5 :P.
http://www.ultra-gauge.com/ultragauge/