User blog comment:Xilinoc/Ch. 557 - He Already Set His Life Aside/I Already Threw Away My Life/@comment-9096222-20131113152848/@comment-5398287-20131119221422

M, I believe it's been stated in the series at one point or another that Ryujin Jakka is as powerful as a Bankai. If not, well, it's fairly obvious even without it being stated.

And as for your Kenpachi postulation, well, yeah, he's one of very few Shinigami who don't need a Bankai to win their fights. Besides, he couldn't possibly achieve it after just learning the name of his Zanpakuto (SHUT UP, ONE GUY POINTING AT THE TENSHINTAI) so it's probably for the better that he's a walking boomstick.