I'll go against the grain. In 2009, that fight could have gone either way.
Stylistically, it's not that different from 2015, Pac's skills may have been even less refined in 2009. But Pac was never going to outbox Mayweather. What he would have needed was the speed, power, output, chin, and killer instinct he had in 2009. I remember Pac had Floyd on the ropes a few times and he never bothered to go further than that, he teed off a bit then just reset. In my eyes, he would have been able to maintain the output he couldn't when he was older, which would allow him to outwork Mayweather to a decision.
That said, there is always the equal possiblity that Mayweather outboxes him
we'll never know