Even among CoC users, some users have a higher ceiling than the other CoC users. That's not the point. The thing is there is no specified baseline for CoC users to say non-CoC users can't reach it. Like, for example, there is no telling if Aokiji is a CoC user, but he shits all over most CoC users in their prime. Nothing suggests you can't become a top-tier without CoC. Law is a walking example of a person with top-tier potential without COC. Sanji too is a potential candidate although I think he's likely to get CoC. He's guaranteed to surpass Katakuri, Prime Chinjao, and Doffy even if he doesn't get CoC.