I still don't see how Kenpachi can just go and beat the 5th espada
I'd assume Ichigo's hollow mask was a significant upgrade from his previous power level. The fact that he beat grimmjow means that Ichigo is now on another plane of power, which is established by how easily Grimmy owned bankai Ichigo.
What is really unbelievable is that Kenpachi, having matched Ichigo Pre-bankai, can now beat someone above Grimmjow, just because he used TWO hands instead of one. If that's the case Ichigo was lucky Kenpachi didn't seem to remember kendo in their last fight.
There's nothing that justifies Kenpachi being so powerful in my book. Not that he isn't awesome but the writer is stretching my suspension of disbelief here.