Nobody has ever said anything about what Itachi is or isn’t capable of, we have never seen him fight in his full strength a life and death fight (I hope we all agree that fight with Sasuke was not a life and death fight for Itachi, in fact everything went how he planned it to be).
We haven’t seen any unplanned battle of Itachi’s, I mean a battle which he wasn’t expecting so that we could see how he fights without having everything prepared beforehand.
He never opened to anyone about himself so that we could know what he himself thinks of his strength, we don’t even know how much did Shisui know about Itachi. Itachi’s main strength in the battle is his cool-mindedness (which we definitely can’t say about Madara 😀 ), which I’d count like three or four kage-level jutsu. (
If you have had an argument with anybody or played a chess, you know what I’m talking about.) Having a cool mind while being able to make your opponent angry is an advantage like no other (Kakashi fans know what I mean 😀 ).
Basically we have no idea about what Itachi is really capable of. We know what he can do, and we know he can do more, but we have no idea how much more that is.