This is impossible to debate accurately because Manu has never been asked to be a #1 option on a team, at least not in his prime.
Manu would likely handle the pressure well, but the pressure is pretty great, and with his nagging inability to stay healthy, an expanded role could have limited overall impact due to an ac ulation of too many minutes and having to carry more of the burden.
As it stands now, Allen was better at what he did great than Manu, and was such while being the focus of opposing defenses and alongside some severely limited/flawed teammates, another issue Manu hasn't really had to face.