Naruto: How strong is Itachi without his illness?
Naruto's Itachi Uchiha needs no introduction. Believed to be a villain at first, viewers ended up kicking themselves when his truth was revealed. Despite not being the main character, many would agree that he stole the show and received as much cheer as the protagonists, if not more.
He is one of Kishimoto's best-written characters and is monstrously powerful. However, he did not make it very far into the story. Suffering from a major terminal illness and battling the repercussions of using the Mangekyo Sharingan, he met his demise when he went toe-to-toe with his younger brother, Sasuke.
Naruto: How powerful would a fully fit Itachi Uchiha be?
In the series, Itachi was mentioned to be suffering from an illness, which remains a mystery, as the specifics were never revealed. He had been taking some kind of medication to "forcefully extend" his life. He was also receiving treatment for a respiratory condition that he was diagnosed with, shortly after joining the Anbu.
As he practiced and soon mastered his Sharingan and Genjutsu, the illness took a toll on him by affecting his physical strength and endurance. Hence, that was the reason he had to take a step back every time he used his Mangekyo Sharingan. Not to mention, overuse of his visual prowess led to his sight being badly affected.
From a young age, Naruto's Itachi was a seen as a genius in the Uchiha Clan. He matured faster than kids his age and even before becoming a proper ninja, he had already surpassed a number of adult clan members. His father was incredibly proud of his son's immense potential and regarded him highly.
He awakened his Sharingan at age 8, joined the Anbu and became Captain at age 11. His training with Shisui further boosted his abilities and was carving him out to be a monster. Following Shisui's death, he awakened his Mangekyo Sharingan and truly became a monstrous shinobi.
Even in the Akatsuki, he was regarded as a shinobi with immense power. One of the Sanin, Orochimaru, tried to take his body but failed miserably. He was able to do all of this even before hitting his peak as a ninja when he was still just a teenager.
Hence, Itachi's potential knew no bounds. If he had gone on to acquire more power such as the Eternal Mangekyo Sharingan, he would have undoubtedly become a menace.
Itachi Uchiha at full power
To put it simply, Itachi would be nearly unbeatable if he was fully fit. It is not known exactly when he contracted his illness, but for most of Naruto, he was suffering. He fought Kakashi Hatake, one of the Leaf's top shinobi, in a weakened state. He even dominated Sasuke while being heavily physically impaired and nearly blind.
With just the Mangekyo Sharingan, Itachi's powers were frightening. If he managed to acquire the Eternal Mangekyo Sharingan, his abilities would skyrocket to a different level altogether. With it, he would gain access to Flame Control (Kagetsuchi) which would boost his Amaterasu. He would unlock a perfect Susanoo.
Additionally, Itachi's Susanoo possessed the Totsuka Blade, the Yata Mirror and Yasaka Magatama (kind of a spinning Susanoo shuriken) - powerful weapons for battle that best even the toughest foes.
As mentioned, he was extremely powerful in his teenage years, even before reaching his peak. Had it not been for his illness shortening his lifespan, Itachi would have grown to surpass the greatest of shinobi and would have totally redefined levels of power.