Koby, in my opinion has one of the biggest potential for growth in the entire series. He went from being an out of shape, non combatant, who probably didn't even know how to throw a proper punch to learning at least one Rokushiki technique in the less than a few months. His aptitude for learning how to fight is insane. I'd even go as far as to say he's a prodigy of a higher caliber than even Rob Lucci, as I wouldn't be surprised if he's mastered all 6 Rokushiki techniques at this point.
Considering he had already unlock Observation Haki, even before the timeskip began, means he had a leg up on mostly everyone else even going into the timeskip. No time needed to be wasted on him unlocking Haki, which means more time being spent on training it. I believe even in Volume 1000 (Which released with Film Z) Oda made a comment of how much Koby's Haki had advanced.
Also, I assume that S.W.O.R.D only takes the best of the best, so if I had to wager a guess I'd assume he's at least at the level of a low/mid tier Vice Admiral, despite only being at a Captains Rank. Is it enough to currently be able to beat Boa Hancock? I'm not to sure, but what I am sure of is he is not far off from being able to achieve something like that.