That's hard to say. Tosen was Komamura's good friend to he might have held back without realising. I mean, it is hard to fight a friend to death. In addition, according to Tosen himself, Komamura is not used to counters. I.e. Komamura is so strong, that he usually defeats his opponent in one blow, specially in bankai. As a result, he is vulnerable if his opponent survives the first blow and counters.