I was answering the question to the poll on if Katakuri will grow stronger in the future and I say yes there, but I go on to say that he'll never be stronger than Sanji. I say this because I'm of the belief that Sanji will be equal to or stronger than Yamato in future arcs as it parallels how Oden went even with Scopper on Roger's crew. And I know for a fact Katakuri won't be stronger than Yamato based off what we saw her do on the roof.
In fact I'd argue Sanji currently probably beats Katakuri. Not only does he have invisibility, (which I believe can somewhat help against future sight) but he also has an affinity for observations haki. On top of that his raid suit is extremely durable. Add the fact that Sanji himself can take a lot of damage as well. Not to mention Sanji is also capable of burning Katakuri's mochi (mochi isn't resistant to fire as we saw). Though I do see Sanji and Katakuri at about the same level. I believe Sanji will beat Queen easier than most people think and go on to help the crew against Kaido later on in the arc.
Now that's just current Sanji, we still have more to come from Sanji with his body acting weird putting on the raid suit. So that's another upcoming power-up, whatever it is. Then to top it off I know Sanji will get to a point where he uses conquerors in his attacks in later arcs.
Katakuri is simply the Vergo of his arc. Insanely strong for his time, but just a stepping stone. He was beaten by a Luffy without conquerors attacks, without ryou, could not hit a base Luffy with his drill mochi attack until Flampe helped him.