The thing about itatchi is he was overpowered to begin with so the illness balanced it out to a certain extent. Take away the sickness with a cloak and he does become a major factor. Fighting against obito with little bro and naruto he wins, the sword and shield put him over the top. Obito would be looking over his shoulder the entire fight waiting for itachi to pull out the sealing sword. Just him having it makes him the ultimate threat, its blade travels long distances and is wide. Madara on the other hand would be a little different. He to would have to pay more attention to itachi but he has more natural tools than Obito. And as far as itachi verse saske goes, itachi does have more battlefield exp than saske but saske was further trained like naruto by one of the three senin so hes deff not a slouch. Imhop itachi got more out of his eyes than saske has but we havent seen whats in store for him yet. Its a good question but a character like itachi was meant to die, hes op'ed, he dies fighting his brother who he was protecting(including tthe village). Dude had a hard life but hes gone let him go. If he was alive the manga would be messed up cause kishi would have to dumb him down and no body wants to see that.