Remember Kabuto took the soldier pill which doubles his chakra when he was fighting Tsunade who was out of commission becdause of her phobia. If any of the Sannin fought him at full strength he would lose in a heartbeat and if they took a cheap soldier pill it wouldn't even be a fight. He's on equal level with kakashi if he was significantly stonger he logically would have killed instead of letting thwart Orochimaru's plans. He does have talent though and is one of my fave characters but he can't touch the healthy Sannin.