It pretty much took the entire Manga for Kakashi to surpass Itachi when he gained a Double Mangekyo sharingan. I honestly thought that Kishimoto was trying to insinuate that Kakashi would have developed faster after his encounter with Itachi's clone in the forest but I was wrong. It really say's something when you compare him to the Sannin, Kisame and other people on Kakashi's level.