Oda has intentionally made it impossible to power scale Koby.
Koby could easily win or lose here, it's totally up to where Oda wants to take the story.
I think Koby asked Boa to join Sword, in exchange for the protection of her people, and pretended to arrest her, only to let her escape somehow, or place her in a prison they can free her from in the future. It's hard for me to picture two of Luffy's biggest fans fighting each other until one is on the ground.