I think he would've been even better at SMW than MW because he would've put on more muscle and not have to drain as much. Not that he struggled to make weight but just not having to hold back physically would've been better for him IMO. Remember when he went from MW to LHW against Tarver? He looked amazing. And that was in 2006, when he was 41. So at 35, around 2000, maybe SMW would've been his best weight.
He clearly had the frame for it and the skills and athleticism in the early 2000s to thrive. Calzaghe would be the only fight he could possibly lose, although I would favor Hopkins strongly. Ottke, Thobela, Catley, Hilton, Lucas, Beyer, Mitchell, Mundine? Hopkins beats all of them going away.