I enjoy talking boxing, so please consider my retorts discussion, not argument.
1) Pac's power is vastly over rated (by the GP) He's not shown power since KOing the vastly over rated Ricky Hatton over 4 years ago. And Hatton walked straight into the punch, his momentum adding to the impact.
2) Pac has faced some of the worlds best, some past their prime (DLH) and some tailor made. Guys like Barerra, Morales and Marquez have good defenses, but still, they are aggressors and fight off the front foot.
3) Pacman, in the bulk of his fights, has fought just 1 fighter that was a defensive fighter. That was Bradley (who has since become more aggressive) and Pac lost that fight. Yes, Iknow he shouldn't have, but he let the powder punching Bradley stay in the fight and it cost him.
4) Mayweather makes Bradley look sloppy, compared defensively. I do not recall the lefty you refer to that gave Maywehter the problems you describe. But believe me when I tell you bro....Manny is tailor made for a slick counter puncher like Floyd (God, I'm making myself sick talking good about Mayweather). That "sideways stance" you mention is why he is undefeated. His shoulder roll is masterful, and seldom will you see him absorb more than 50% of any single punch.
5) Southpaws have very little advantage over a seasoned orthodox pro. The biggest being rightys don't see many leftys but leftys see many rightys.
But a righty is positioned to land with power, against a southpaw, almost non-stop (If you fight, the next time you face a lefty, feint the jab, throw the straight right. It will be there). Pacquiao will look to throw a similar straight right at Mayweather, but that will be countered by that damn shoulder roll and the one thing that really sets Mayweather above the field, is his uncanny ability to space himself to his own advantage and to his opponents disadvantage.
6) Mayweather often baits his opponents by lowering his left when he throws his right. He will do this 3, 4, 5 times while staying out of harms way, quick to get back to his back foot. The opponent starts getting comfortable seeing this, and strategizes...(and Floyd knows his plan before he does) the next thing you know, they make their move (having to lunge just a little to get there)... that's when Floyd steps out and comes over the top (of that lunging straight right) with the hard left cross. And the announcers pee themselves every time. :/
Manny does his fair share of lunging (anyway) and this won't bode well against Floyd. Pacman only knows one method of fighting and it's why I love the guy. He is a volume puncher that comes forward. This my man, will play right into Mayweather's hands. Pac is no better than Marquez, in fact, IMO, not quite as good. Mayweather beat Marquez with ease and how I hated that match to begin with. :/
Should it miraculously happen, please don't buy this fight. Floyd does not deserve your money. And if you have to bet... parlay the over while betting Floyd by decision. I've seen this fight 100 x in my head....always with the same outcome and have no clue why Floyd hasn't jumped all over this. "Roids" be damned. It's just bizarre.