User blog comment:Xilinoc/Ch 649 - The Theatre Suicide SCENE 3/@comment-9282993-20151030083642/@comment-5398287-20151101064534

Well, let's see. His Bankai is composed of four completely uncounterable and unavoidable stages which are each progressively more damaging and despair-inducing, concluding in a throat slash that blows up his opponent's head, a wound which probably only Lille and maybe Yhwach can survive because no one else has displayed regeneration abilities on a level that reaches immortality like that.

So yes, even though Shunsui is the Captain-Commander and such power is to be expected from him, his Bankai is unquestionably powerful, to the point where someone may designate it as OP.