How mayweather fares against pac it's really hard to judge.
IMO Pac looked absolutely terrible against Mosely. The in and out rhythm, movement, angles wern't there. The speed and power was. But this needs to be complemented by his amazing footwork, rhythm, and angles for him to be really effective against these bigger men. The footwork we saw against margarito was phenomenal. It was not to be seen against mosley.
Pac said himself that it was the result of having issues with his legs and cramping. I'm personally not sure whether it is simply this or instead indicative of him in decline. I certainly know his style isn't one conducive to longevity. He is getting rather ancient for a man with this style who is from the lower weight classes.
Fighters have off nights. Shit happens. His fight against Marquez will confirm whether or not he is experiencing significant physical decline. If he is losing his legs i see him really struggling against Floyd. He will be a fighter reduced to fighting flatfooted, lunging opposed to seamlessly moving in and out of range, trying to land powershots. With sheer speed and power alone he has a chance of knocking Floyd right out but i might have to Favour Floyd if Pac shows such signs of decline in this next fight.