Definitely not, the Welterweight division is rich with talent, just stay there and get involved in some major fights.
If Williams can keep making weight he absolutly should stay at weltereweight. Even without Mayweather its still one of the deepest divisions in boxing. 154 is a graveyard of a division. DLH is almost done, Forrest just loss to Mora, Dzindurk may never leave Europe. His options at 154 are basically Mayorga, Spinks, and Alcine. Those fights don't actually look too enticing. If Williams is going to move up I say go to middleweight. I don't think he'd beat Pavlik but its a lot better option than anything at 154.
Williams should be campaigning at 154. I think the stoppage was premature and should give Quintana a third fight at 154.
It depends if he isn't having trouble making 147 "why do it?". He really did great stopping Quinatana in a rematch. http://www.sportaphile.com/2008/06/...olishes-carlos-quintana-first-round-knockout/