I feel that if booing does in fact make him weaker, that would make his defeat easier said than done. There has to be some sort of catch. Maybe it'll only reverse the process of becoming stronger from all the positive cheering and return him back to his base form.
Anyways, I think that negative cheering would only work on him if the one doing it is one of his subordinates, namely James in this case since he was his source of power to his Vollständig abilities. If negative cheering does in fact make him stronger (at an even increased rate than what positive cheering would evoke), then I feel that James would've done so sooner or later before facing certain defeat. But Kubo showed us none of that. We can only assume and speculate. Again, this is only speaking if the theory above is true.
