First thing first, I've suddenly notice a hugh influx of thread of random people bankai.... even Hanatouro . Anyway In hope to reduce useless topic. I'm makin hopefuly this final Bankai thread in a while.
Now on with the topic. Currently there are 6 shinsegumi that was sent to earth to help out Ichigo. Renji, Rukia, Ikkakau, Yumi, white hairy captian (can never spell his name, and Masumoto. I'm here to offer some specalation on why do everyone seemly so powerful.
Renji - Obvious why he was sent on this mission. Being one of the few non captian to able to achieve bankai.
Rukia - There many arguement that bitch that Rukia power up. I don't really think so though. She always had the potential. (At your age she seemly surpass Renji at Demon Arts) The whole reason why she seem so weak was guilt and inferiority complex cause by Byakuya. Since the whole SS arc the truth review, and finally properly apolizing to the shiba clan, yadda yadda, she finally able to regain confidant. I also say she agian not as powerful as vc or captian, just because she beat dee roy easily. In just the fact that her ice pillar attack completey caught dee roy of guard. Doesn't matter how strong you are, you still can get kill if your over confidiant and not expecting a attack. It like me coming over behind you and knifing you. You can defend yourself if you don't expect it.
Ikkaku - Until the new chapter is fully translated, I can only offer a specalation on why Ikkaku have bankai. I actually think he always had it, even when he fought Ichigo. Not only that, I think Ikkaku been a shingegumi alot longer then the renji/rukia generation, so if that the case the above is true. However Ikakku is like a bald version on Kenpachi, love to fight. Therefore he'll fight with the person with a disadvatage. I also believe that although Ikkaku has bankai, I don't think it a "true" captian bankai, like renji. As Byakuya said in his fight with renji, it will take another 10?, 100? years of training to fully master it.
Yumi - He probaby in the same shigegumi generation of Ikkaku, so he already an experience and devolop fighter. Although he not at Ikkaku level yet. At the moment, All I think is that he just tagged along. However, I don't think he a weakling. We know he SSlayer ability can drain reiastu, which at any level, seemily dangerous.
Higsturgira? - whatever his name is, I don't really like him. Obviously a captian, obviously a prodigy, obviously powerful.
Matsumoto - VC level. But think about it, she probaby enter the shinsegumi at the same time as Gin. Gin obviously a captian level. Base on that, Matsumoto probaby been around for a while. At least long enough for her to devolop her skill to captian level at most. Althought I won't think she that powerful. Altought I don't think so, I wouldn't be surpise at all that she would be able to use bankai, she obviously have more then enough time to reach that level. Even if she has not, she probaby powerful with her shikai alone. (We don't knw what it do yet, but we do know he get into a gasous state. I don't knwo how you block something like that.)
I will probaby add more later... but i got lazy typing.