Actually while reading through the mangas again, I noticed something Tsunade said. She said in Chapter 164 page 12, " This boy is not an ordinary medical ninja. His senses and strength may exceed even my own in my prime." She has all but conceded Kabuto's superiorty as a ninja.
Sorry to quote myself, but Tsunade said he is more skilled than she was in her prime. I dont know if he could defeat Orochimaru because he swaps bodies and mantains the strength he had in his prime. But it think its clear that his power excedes that of Kakashi, unless Kakashi can in fact defeat Tsunade. Just my opinion.