Forrest would punish Pacquaio. Forrest was too good a technical boxer. Too good of a jab, too good of a right cross and too good of an uppercut. He would have a huge height advantage, and reach advantage. Pac would probably be stopped. It is humorous listening to people exaggerate and over hype Pac's ability.
the thing you have to take into account here is the pac and even mayweather could very well struggle with some of the past great welterwights because they are SMALL welterwieghts who started out at low weight classes. in recent past, you had big welterweights like trinidad, de la hoya, quartey, forrest, and mosley to a lesser extent. they not only were strong at 147, but damn good fighters. you dont have great big welterweights right now.