I mean, Yuji's attacks cause Mahito's body and limbs to explode. Not to mention in any fair 1v1, Mahito gets absolutely decimated by Yuji and always ends up coughing blood and trying to run away. The only way he ever manages to do damage is through cheap blows and shapeshifting his body into more dangerous (usually sharp) weapons, or tricking him (by using people as bait). The fact Yuji also endured a beating from Mahito while not fighting back at all, including a Black Flash with comparatively minimal damage is further proof of that imo. Meanwhile Yuji's Black Flashes always completely decimate Mahito.
Yuji is far beyond the standard for a Grade 1 from the very fact he can fight Disaster Curses at all considering even newborn ones are too strong for most Grade 1's to fight even in 3v1s. Let alone 1v1. So him being a bit stronger than the baseline of a Special Grade (Which tbh is prolly Geto. Geto is kinda weak asf, def the weakest Special Grade, and physically he ain't that crazy. Kenny really made him that much better) is fine imo. Not to mention the fact that Yuji literally combated Yuta in the next Arc when he's not at full power, and yet Yuta still complimented his strength. And Yuta is a very high Special Grade Sorcerer. So I'd say it's fine imo.