Aside from regenerating, shouldn't he do worse than SSJ3 Goku? Wasn't Goku slightly stronger than Fat Buu before getting tired? Although I don't know if Fat Buu was taking it seriously, since he's an idiot and he knows he has regeneration.
He'd get slightly beat up, but would make Kid physically work for it and would do some damage on the way, which would force Kid to use energy regenerating (but not as often as Fat would have to), but is there any way of knowing how quickly either Buu would tire without their infinite stamina?