We've never seen healthy Itachi. It was stated that he was already sick since his first fight against him in Naruto, and he still owned him. A clone of him also owned Kakashi, Naruto and Sakura together in Part 2 (remember)?Sharingan Kakashi vs Sick Itachi= Kakashi High diff
Sharingan Kakashi vs Healthy Itachi= Itachi mid-high diff.