I agree kenpachi wouldnt stand up to aizen because his ability to withstand cuts is because his opponents have lower spiritual pressure but if Aizens such a bad ass as he was made to be he obviously has high spiritual pressure.
It makes sense that in a society like Hueco Mundo where they only respect power that Aizen and his "generals" would be more powerful. Which makes me wonder to if he really needed to go through all of this to complete his goal the guy hasnt even released his zapakto and he has already defeated 2 bankais. If my theory of him using the Hogyoku to either gain his resurrection hollow powers or to be able to change a vasto-lorde who I think might be the only espada standing in the real world is correct did he really need that much fire power? Heck the Vizards captains are the only ones who have the ability to fight him (if they revealed there true power which im not satisfied they have done) and they wouldnt have that ability if it werent for his great lengths to get those powers