Sanji getting CoC at all, much less AdCoC, creates all kinds of issues with the concept of CoC being the highest form of willpower.
It would be fine if Oda had not decided to show Sanji faltering on multiple occasions.
Reasons why Sanji should logically disqualify to ever getting CoC:
- told Zoro to discard his own dream if it would otherwise cost him his life
- did not pursue his own dream until Luffy showed up to push him in the right direction
- left the crew because he doubted his own and their strength
- accepting to join the crew on Luffys terms instead of his own
- while being central to his character his dream is not related to willpower
It would take a lot more than an old axe dual wielding, giant shagging lecher with CoC to convince me that Oda is willing to piss on his own requirement of unwavering will for characters that get CoC.
Depending on how you view the character Katakuri was maybe already an outlier for CoC, which leaves room for interpretation in Sanjis case.
imo, it would certainly suck and devalue the ability massively if Sanji were to get CoC.
And it would suck even more if Oda then decided to add another hidden ability that is even more special just to compensate...