Seidou has the organs of a SSS ghoul and he's still half kakuja SS... lol...
Kaneki Ken is the true masterpiece of Kanou as Kaneki got the organs of a S rated ghoul, but at that time, his strength was equivalent to a B or A rated ghoul. Kaneki then climbed up to a high S rated, then high SS rated, stayed like that for a few years, but after truely awakening, blood hair Kaneki is now SSS rated, eto was probably trying because eventhough she wanted to see how Kaneki might progress, she never let anyone inflict so much damage on her before. I mean ffs Kaneki chopped her in half, her real body while arima only cut of her kakuja's arm lol. I think Kaneki Ken might be a more formidable opponent against Arima because Kaneki is seriously fast to the point of having dodged all of Eto's attacks which she specifically said "let me kill you." ghouls like Shachi, etc. would've been pulverized lol...