I feel that's hard to determine given that he only used his final Susano'o with killing intent against Orochimaru and Nagato, neither of whom lasted more than a few moments. And even then, he seemed like was being pretty casual with his trumpcard. We know he can move and whatnot with Susano'o out, so he could've blitzed like he did against Bee for additional pressure if necessary.
He also has hyped mass genjutsu that we've never seen, so far-reaching and powerful that it's evidently able to have a huge impact on a world war. Combine that with his ridiculously hyped intellect and the fact that he's always holding back for some reason, and it's hard to really get a gauge on his real power. Like I said in the OP, the only people that can be said he truly wanted to eliminate were Orochimaru and Nagato, and neither was able to last long when Itachi used his best ninjutsu (final form with legendary items.)
