"Wait... I don't really get it. Why does gluing on a CF roof help add torsional rigidity? A steel roof doesn't float too, would that give the same strength as gluing a CF roof on?"
I didn't mean to imply that a glued-on CF roof would have more torsional rigidity than a welded-on steel roof, only that if it were not glued-on (or rigidly attached in some other way) the body would have less torsional rigidity.
At the start of this thread some were speculating that the CF roof was just a sheet of CF in a metal frame and that it imparted no structural rigidity at all.
I don't know whether the torsional rigidity with the CF roof is more or less than that with the steel roof but certainly the CF is lighter. And as above, glueing it in place puts back some, all or more of the torsional rigidity from not having the steel roof.