When Aizen stated that there were limits to the powers a shinigami could obtain, I believe he was referring to his potential "army". Even though he could have easily turned many in the soul society, and create an opposing force consisting entirely of shinigami. The force would never have been a match to his current/past espada.
As arrogant as he was, he would never imply that he had limits, or wasn't strong enough. I don't believe that this in itself was a reference to him gaining/having hollow powers. Instead it was about the growth potential of hollows. He could never gain that many captain level shinigami, that quickly. Plus the complete freedom of Heuco Meundo in which to work and experimant on hollows. Any thoughts?