AMG's original ducktail was custom steel fabrication. The add-on piece they made was only for pre-facelift 500E and would require bodywork to conform to the different profile of the E500 trunk lid, although not sure how that would work with the injection-molded material they used (someone must know the correct term). The AMG copies I've come across also have the same issue so if you find one that fits the facelift trunk it would be welcome news to many I'm sure. As to your question, I recall from the early articles on the Hammer that the distinctive heckspoiler provided meaningful stability at high speeds so, looks aside, it has a functional role for those inclined to vmax ;-)
Years ago I ordered one at great expense from AMG NA, but sent it back because of the fit issue. Recently came across the badge kit that came with it, seems I forgot to sent that back!