I think they would have to upgrade the power output, because it has enough trouble moving the hard-top at a rapid rate, and with the extra weight of all of the mechanisms in the Convertable, I think it would actually match the power and all forces would be equal, and I don't think it would move at all. Still you would look cool sitting on your drive with the roof down drinking beer and eating burgers.
What I'm saying is, please BMW, upgrade the power to 100bhp and increase the torque, and then make a convertable, then us hard top people can have the engine as well and have a little more fun accelerating faster, e.g. Skoda Fabia vRS
style.