Exactly at what point in the series has Itachi has gone all out in fight? Was it when he teamed up with Naruto and Bee to defeat Nagato or was it when he teamed up with Sasuke to defeat Kabuto?
Clearly when he fought Sasuke when he was alive because that was when he revealed all of his powers like Amaterasu, Tsukyomi and Susano except for Izanami of course, maybe he couldn't use it then due to lack of Senju DNA, please see this for more info >
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. Although, that time may not count because Itachi is supposed to have not been trying to intentionally defeat Sasuke.
True strenght lies in not needing to use it. Itachi doesnt go for a stamina fight, he likes to incapacitate/defeat his opponents regardless how much chakra they have left.