Paul Williams? I mean honestly, looking at it, he'll get major props from boxing fans all over, he'd have an epic fight on his hands and if he were to lose, he would lose to a 6'1 beast who never gets acknowledged by the big time prizefighters at welterweight. Win/Lose, Pacquiao gets mad props for fighting Paul Williams, and on top of it Paul lacks defensively, Floyd is a nightmare stylistically for Pacquiao, lets face it. And who knows, Paul hasn't done 147lbs for a long time, he may have to drain himself to do it now, but he'd drain himself for a fight with Pacquiao.
He would only get that recognition among hardcore fans, casual fans would not take all of what you mentioned into consideration as they don't know enough about Paul, so would just see it as a defeat. I would say that what you said applies to Mayweather as people would not take anything away from Pac if he lost that fight, they would just put Mayweather on a new level.
People will lose interest in Pacquiao once he loses to Floyd. I think Pacquiao - Williams would do real good, fill cowboy stadium with that one and not only will the people get their moneys worth, Pacquiao would still have options win/lose, options that would be lost if he got beat by Floyd.
