No sickness, no blindness and no reason to hold back at all? Just up and kill whoever he faces? Then we can use the novel which is canon to the manga. Itachi used Tsukuyomi on thousands of insects at once and then proceeded to burn the insects and their users with Amaterasu WHILE Tsukuyomi was in effect. He also used Tsukuyomi on two 3 tomoe sharingan users (Izumi) and the other was an elite in the police force and his fathers very own right hand man(Yashiro) and neither of them could get out, no matter how hard they tried, one of those two he chose to mercy kill with Tsukuyomi making her brain shut down, while if I remember correctly he proceeded to burn Yashiro with Amaterasu. It showed him clearly able to operate outside of Tsukuyomi, able to burn both the Aburame and all of his insects while the insects were all caught in Tsukuyomi. Itachi has other feats in the novel too. Mind you these were all at the age of 11-12, because it is revealed in the novels that one has to be 13 to be an Anbu Captain and Danzo and Hiruzen each agreed together to make Itachi's public age 13 and make him an Anbu Captain at 12 years old.. so Itachi was that strong at only 12 years old. He gains his illness after joining Akatsuki, but before he shows up in P1 as according to Obito he appeared after Hiruzen died in order to show them that he was still alive, he was also Konoha's only spy inside of Akatsuki and it is heavily hinted his intel was given to Jiraiya anonymously who went on to give the intel to Konoha's higher ups. Itachi in P1 and P2 never kills anybody nor ends up using his actual full ability on anybody even when he was near blind and near death against Sasuke, as Obito says Sasuke didn't actually see through a single one of his illusions and that if Itachi actually wanted to kill Sasuke he would have. In the novel I forgot which page, but somebody says that Itachi(a 11-12 year old kid) could have been Hokage or something along those lines and you have to be a certain level of strength to be Hokage.. that means stronger than everyone else in the village except for maybe the other candidates. Going back to the Itachi v Sasuke battle even BZ says something is wrong with Itachi and his speed and reflexes should have been much better. So a hint we get of Itachi's abilities is him in the 4th War, I say a hint because 90% of Edo's are just weaker versions of their living selves that get unlimited refills of their albeit smaller chakra reserves. Proof is Madara going from having both Rinnegan and being stuck in the Great God Gates by Hashirama, then him coming back to life and having no eyeballs but breaking out of the Great God Gates with ease showing an immediate and noticeble difference from Edo to living again. If Itachi was brought back to life with no illness and full eyesight his overall gain should be the same as Madara's Edo vs living overall gain. We saw Itachi casually parry KCM Naruto and dodge Killer B at the same time basically when B and Naruto's goals were to first talk to them(failed since Kabuto took control) and then incapacitate them so they can talk to them, which still basically failed). B couldn't land a single hit on Itachi and Itachi was able to crow clone switch with B having no idea it happened, Itachi also landed him in a Genjutsu despite Gyuki being present. Naruto STILL warns B that if he gets hit with Tsukuyomi it is over knowing B was a perfect Jinchuriki. Basically Itachi is underestimated by a lot and overestimated by a lot. Though he isn't defeating Hashirama/Kaguya or the other Otsutsuki/4th War SPSM Naruto-Hokage Naruto/4th War Rinnegan Sasuke-Adult Sasuke/8 Gates Guy.
