Uchiha Yi had no desire to fight beyond his limits in Sage Mode the way Hashirama Senju had. The human body had its limits, whether it belonged to Hashirama or Yi. Until he became one of the Ōtsutsuki Clan and left his humanity behind, he needed to find ways around those limits.
With Jūgo's help, Sasuke had entered Sage Art: Susanoo. But Jūgo's Sage Mode was an extremely crude version. It had merely let Sasuke grow stronger by using Senjutsu Chakra; it had done little to increase Susanoo's power.
That was all the situation had called for. At the time, only Sage Art could harm Obito Uchiha after he became the Ten-Tails Jinchuriki. Sasuke Uchiha had not needed to worry about how much stronger it made him. Being able to use Senjutsu was enough.
"Oh, right, Yi. Tsunade has run into trouble. She may need to summon you over to help later," the Slug Sage said suddenly, one of her enormous feelers swaying.
"Tsunade's in trouble? Has something gone wrong at the front?" Yi was startled. He did not care whether going to the front without the village's permission would cause trouble for him. Grandma Mito would handle it.
Besides, his strength in his normal state already surpassed that of an ordinary Kage-level shinobi. With Sage Mode, Yi considered himself stronger still.
As far as the Shinobi World knew, only Mito Uzumaki was stronger than him now. But Mito was his "teacher"—his grandmother, even if they were not related by blood.
With Mito behind him, he had nothing to worry about. He was also publicly part of the Hokage's faction, and he would be going to save three of Hiruzen Sarutobi's disciples. Hiruzen would have to step in and take the heat for him. Otherwise, his followers would lose faith