But regardless its just really hard to see Coby getting to that level considering how weak he is right now.
My guess is that it will be a two part power up. Teach offers him a spot on the crew and a DF, Koby eats the DF in order to prove his loyalty. Then in his final fight he gets a haki awakening. Probably some god tier CoA that Garp had.
In any case, Koby is different from Kid and Law, he wasn't thrown in due to an editor's request, he's been around since chapter 2. Furthermore, his parallel to garp indicates his importance. Garp wasn't merely a rival, he was one of Roger's closest friends. It was Garp that roger entrusted Ace too after all. I think Koby is similar in that he will be an "unofficial SH".