You are actually right if you don't include berto. Berto is number 3 in the division and has the WBC.
Im not even sure Berto is all that relevant tbh. Accounting to most in the States he is an HBO hype job. IF Pac was to add Mayweather's name to the list of Cotto, Clottey, Mosley and Margo (I was it was at 150 but he can still be considered a welter), the once mighty WW division would be cleaned up. The berto fight wouldn't even be interesting as I think he loses to all the guys Manny has beaten
Also what is the definition of cleaning out a division? Bernard Hopkins was known as the undisputed champ and I believe he had all of the belts. To some people if Pac beats Mayweather and Mosley he still didn't clean out the division because he didn't have all of the belts. I agree with you though. If he beats Mayweather and Mosley (possibly Berto) then he pretty much cleaned out the division.
If he beats mayweather, he cleans the division, its as simple as that! But its a shame it wont ever ever happen