Well, but what exactly would kyoraku do if he does figure out gin's bankai? What good is that knowledge? The bankai has insane range, speed and a one hit sure kill. Realistically speaking the use we saw giving his bankai during the first moments of his fight with ichigo is not at all what that is about. How exactly would shunsui deal with gin's bunto attacks with shikai alone? I don't think he could in the long run. All gin has to do here is keep pointing his sword towards shunsui. There is also the consideration that gin did not actually want to fight ichigo as far as we know, that won't be the case here.
Well, knowing it would at least make him sure of one thing: He has to dodge. He cannot expect to get hits and manage to survive.
I do agree, however, that Ichigo fight isn't something to go by. Gin was one of the trickiest opponents to fight against in the entire series, so, when he had zero killing intent whilst fighting Ichigo, that meant little.
Both have the potential to gain a massive advantage with their powers, but I'm going to give it to Shunsui then. Just because if they spar with their Shikai for a bit Shunsui will be at the advantage. Neither are the sort of person to go all out straight away, they tend to fool around a bit since they know how powerful they are.
Still, be a very interesting battle nonetheless...
He is not actually saying shunsui's shikai abilities are more powerful than gin's bankai. He is referring to the set of games shunsui has that would stand out from the rest rather than actually comparing a bankai to a shikai.