He only defeated a weakened Orochimaru, and he took 3 years to study all of his powers in preparation. Kakashi could have easily accomplished the same feat if he had that much time and ease of access to Orochimaru's skills. When Sasuke got MS, he stopped thinking out his moves before making them. He was nearly killed 3 times alone in his fight with Killer Bee, and that was with his MS. Plus, he overused the powers of his MS and promptly blinded himself. Now that he has EMS, he has been showing a little more logic in his fights, he has a complete Susano'o, and hehas perfect control over the Amaterasu. I think that he only surpassed Kakashi after acquiring EMS, because that's when he began using his powers efficiently and logically rather than rushing into battle and wasting all of his chakra in the first few moves like he did against Danzo.