This has been done to death, but here goes. The only way Floyd could win would be to run and pot shot his way to a decision. Would not happen. SRL was faster than Floyd, and would be able to stay right on top of him. In addition, Floyd couldn't buy respect from a beast like Ray with his life savings. You're talking about a man who took Tommy Hearns' best shots and kept coming. SRL forces the fight all night long, seriously hurts PBF somewhere in the middle rounds, and puts him away. Of course there is a very good chance that both would die while waiting for the other to get old!
Floyd should be 0-41 the way he is being discussed in this thread. That being said, yea, SRL would be a tough matchup for Floyd. I'd probably lean towards fairly competitive UD for Leonard over a 12 round fight P4P.
Sugar Ray Leonard, in his prime, was one of those fighters who always had a game plan to support his exceptional skills. He has the work rate to outpoint Floyd easily. Floyd is very accurate, but not a busy fighter. Ray was too big and too good movement for a realistic Floyd knockout. There is no evidence Floyd could knock him out, hence he did not even knock out Mosley or the slower much easier to hit Marquez. Another part of this is that Sugar Ray Leonard was a beloved fighter. If the fight was even close, he gets the nod from the judges over the villain Mayweather Jr. One other thing to consider is that Floyd has the benefit of todays more technologically superior training equipment while Sugar Ray was old school. Would have been interesting if todays equipment were available for him. Remember that Sugar Ray Leonard came from the days of 15 round fights. Floyd Mayweather is NOT a knockout artist. He is a tactician. Very, very well schooled and skilled. But, he has not faced anyone with nearly any skill like SRL except a beyond prime Oscar de LaHoya, and even that was a close call. If you did not grow up seeing SRL, then you could never justify your answer to this question.