I've read that the coupe and sedan have equivalent performance.
I would definitely stay away from the soft top as a track car, as many clubs and tracks prohibit or restrict convertibles. Current BMWCCA rules in my area (NJ and DelVal, PA) prohibit ragtops on track unless you have a full cage and a log book. Some tracks (NJMP, Lime Rock) have fairly restrictive requirements for convertibles as well.