@Red Admiral
Ray said it. While you can gain control over your CoC, you can't train it. It becomes stronger along with your growth. Shanks is extremely strong and ambitious, his CoC reflects that. Same should be true for Mihawk, who had the ambition to become the strongest. And especially Zoro, who is not only as ambitious in regards to becoming the world's strongest, but to aid Luffy in becoming the PK.
So there isn't a reason to put Shanks' CoC on another level, unless you say his ambition and strength is above everyone else.
Kaido for example is strong but he doesn't have much ambition. He sleeps on his DF.