What boxers would you say gained weight for the better, by stopping weight cuts etc? The one that comes to my mind is Pedroza. KOed by Zamora, he moved to featherweight, and you know what happens next. He then became greater than his lower weight conquerer through his dominance.
RJJ and Ezzard Charles are the first two that come to mind, but probably moreso that both were inexperienced at 160 and gained experience as the added muscle to their frame Greb seemed to do alot better at 160 than 147. Probably the same lack of experience and athletic development reasons though Oh the poster boy for this is Evander Holyfield though
I have to put forward Manny Pacquaio. Manny's physical strength/punch resistance/stamina/punching power all improved after moving up from flyweight. People sometimes forget he was stopped twice (once by a bodyshot) at 112lbs. In moving up several weight divisions and facing several big punchers, he's only been stopped once since (ko by 6 Juan Manuel Marquez). He truly grew into his adult frame and went on an incredible run from 122 - 147 (skipping 118). If Manny elected to somehow stay at -122 his results would have been probably mixed and he may not have become the hall of famer that we know today.
I don't think Benitez was at his best at 140, even though he was champion. Of course he was extremely young then too.
The one that jumped to mind for me was Adamek when he moved to Cruiser. Ironically, Adamek's subsequent jump to Heavyweight is the polar opposite.
Archie Moore was better at LHW than at MW. Sam Langford got better and better as he gained weight. Some may argue that Fitz was even better at HW than at MW, though it's hard to say as he was basically unbeatable in lower class but he lost twice to the best HW he faced.
McGirt was better at 147 than 140. Khan better at 140 than 135 Collins better at 168 than 160 Toney was better at Cruiserweight than 175 surprisingly
