Why are you limiting Jinbei's strength? You make it seem as though he just a normal pirate and not a former warlord, leader of the Sun pirates, and strongest user of Fishman karate. He was strong enough that he was kept on the bottom level of Impel Down. Him joining is a huge deal because of how strong he is. When that news gets out it will elevate the status of the Straw Hats with the Marines in a major way.
You mention logic with Jinbei compared to Doflamingo but there is no logic to be had there because we haven't seen them fight. You are artificially making him weak because he joined the Straw Hats.
Jinbei is strong but I would say the same thing if Sanji was able to do something like this to an admiral/yonkou tier character. Being a former warlord means nothing since we know that they all vary in strength, unlike the Yonkou who are all relatively equal in strength. The majority of the warlords weren't capable of becoming a Yonkou or even a commander. Fisher tiger was at one point the captain of the sun pirates, and he lost to a rear admiral :/. Jinbei already declared his loyalty to Luffy in Marineford so I doubt him joining the crew will have a big impact.
We don't need to see certain characters fight to know that one of them is stronger than the other. It's logical to assume that Doflamingo is stronger since he was an arc villain who was almost on par with Luffy.