I'm pretty sure he first reached Miu's level by the D of D Tournament, as shown by how he could fight alongside her as a team. When he unlocks Ryuusui Seikuuken and defeats Shoh, I'm pretty sure that was the first time he surpassed her. Shoh immediately knocked out Miu when they fought, and didn't even have to use Seidoh Gouitsu to keep up with her when she went berserk. Ken'ichi beat Shoh's Seidoh Gouitsu, so that should put him above Miu.
By the Sakaki and Hongoh arc, Ken'ichi seems to be pretty clearly above her; he one-shots Shoh's friend who was capable of hurting Miu, and he could keep up with Sakaki and Hongoh's fight better than she could.
It's not until Miu trains under Jenazad and reaches the second Ki stage (Kaihoh, how to release Ki) that she becomes stronger than Ken'ichi again. But Ken'ichi should have surpassed her again when he reached the third Ki stage (Shouaku, how to hold Ki) and beat Kajima, who was clearly greater than or equal to Miu. By the end of the manga's original run, Ken'ichi's Ki is so intense that some skilled skilled martial artists mistake him for being a master.