I think it's for the best if people keep underestimating him. For me it's part of his badassery. I mean, whenever they fought strong crew/opponents, he was always the "no name" guy, he doesn't even have a proper Wanted Poster, he is just the cook, then he fuckingly surprises them all the time, showing how a monster he truly is (and yet)...To the point he never was in a real critical situation like Zoro or Luffy in his 1vs1 fights. It's like he has still yet to show his real strength. He is like the black swan, he is the unexpected.
Even during the last arc, he had no spotlight until the real fight (he wasn't in the palace when trouble started etc).
Guys above said everything about the Mermaid's island issue. He went from his personal last level of Hell to Ultimate Paradise, that's too much to take.
His main strength is not being expected. That's his badassery. Although I do agree that after the timeskip we haven't seen badass moments of him. I'm expecting them now, though.
So... Now being stuck in Nami's body since last week, I'm curious how Oda will turn that again into actually trolling Sanji big time. I guess he'll have an melée of his inner perv to do nasty things to her, or "just" take a look to what seems to be his -er- hers and his inner gentleman not to harm Nami's body whatever may happen.
It would be interesting to see him resist his urges out of respect for Nami though.