I will be in my new car, but I wouldn't do any auto-x with a w-body.
the 18s are kind of a rough issue though. they will help because sidewall roll is decreased, but they are also heavier. im not sure that youll have a problem with them since you have a pretty well maxed out motor setup with your blower, but its just something to think about. how wide of tires are you running? if i was doing autocross i would shoot for at least 255 series tires. maybe it would be a good thing for you to invest in a nice wide set of 17s and throw some big meats on them? thatd be up to you.