Floyd Mayweather is much older than Pacquiao (although Pacquiao has been in more "wars" in boxing and was probably more out of prime), but there are many reasons why I think Prime Floyd would smoke Pacquiao even worse.
-Prime Floyd was more aggressive. When he was Pretty Boy Floyd, he knocked people out and had a perfectly healthy hand
-Prime Floyd was more faster and had quicker reflexes
-Floyd smoked Canelo and Marquez, who knocked out Pacquiao
-Floyd hit way harder in his prime