I don't believe anyone genuinely thinks Tony would have beaten Mayweather then or now. But it would have been interesting to see how Mayweather would have dealt with an incredibly durable, supremely well-conditioned, relentless swarmer who had the will and determination that few others possessed.
Mayweather had just moved up to that weight class, was relatively small for it at that particular time and it was a pretty compelling fight around 2006-2008. Floyd potentially could have whitewashed him and pitched a shutout, or he potentially could have found himself in a difficult, competitive bout against a man who could have zero respect for his power and who was capable of setting the sort of pace that would unsettle the smaller, economical counter-puncher and favour the bigger, perhaps stronger, high-volume punching pressure fighter.
I'm not saying it would happen for certain, but Floyd wouldn't be the first outstanding talent to face adversity with regards to a fighter who's will is every bit as strong as his opponents considerable skill.