If he drops down to 154 and beats Cotto, Canelo and Floyd, I'd put him in the top 50 ATG. On paper, the records of his opposition would be superior to any other fighter in the world over the last 25 years or so.
Sergio is always going to be one of these guys that people want and expect more from, whether it be from envy, hate or the acknowledgment of his greatness. If he dominates an experienced boxer with 35 wins and no losses at the sake of missing an opportunity to fight a greener, undefeated guy with 15 wins and no losses, who has a little hype behind them, Sergio is "ducking" this other guy. If he beats up on the greener, hyped guy then he just beat up on a green "hype job" who wasn't ready to begin with.
It is the same story with guys that are too good for their own good. You are damned if you do and damned if you don't.