Now, I'm not gonna go so far as to say that Franky is stronger than Luffy, but let's put it into perspective.
After filling up on Cola, Franky was able to break through Fukuro's Tekkai with no problem. Franky's main weakness is that he's a bit slower than most of the strawhats. Plus, getting hit in the back is a big weakness for him.
Case in point, though, Franky doesn't need to learn new techniques (So far) to fight his enemy. That stupid Centaur thing he did with Nero was probably just to show off. With Fukuro, it was pretty much a normal fight following Chopper actually giving him what he needed. All the other Mugiwara had to learn new techniques or undergo some form of enhancement (Luffy's Gears, Nami's upgraded Climatact, Sanji's flamberge Shot, Zoro's wtfasura move, Chopper's Monster Point).
When you look at it, if Franky joins, he'll be one of the strongest Strawhats. In most Shonen Manga, anyone more powerful than the hero usually doesn't join. Yes, in DBZ Gohan was more powerful than Goku at the end of the Cell Saga, but that doesn't really count because Goku massively surpassed him during the Buu Saga.
Defeating someone doesn't necessarily mean you're stronger physically, but in the Shonen World, it does.