This logic doesn't really work though. No one said their fighting styles have to be exactly the same
Roger is a top tier swordsman without a fruit while Luffy is a top tier brawler more like Garp that also has a mythical zoan
I don't see what Gaban using an axe really has to do with anything
Not to mention Sanji's abilities are also reliant on his haki. A combination of the exoskeleton and his armament is what allowed him to create Ifrit Jambe in the first place
Since his exoskeleton (at least as far as we know) can no longer improve then the only thing that could push his flames further is improving his haki
Roger / Luffy - AdCoC , VoAT , Haki Beasts
Rayleigh/ Zoro - AdCoC , Swordsmanship , Haki beasts
Gaban - AdCoC (if) , Ace User, and ?
Sanji - Exoskeleton , Fire Conjuring.
What similarities do you see here?
I'm not even against sanji being Strong but Oda isn't taking that route with Sanji.
We have characters at top tier without CoC.