Webafter turning into a demon he gained a lot of experience fighting like a demon.
But, does that affect muzan in any way?
Does muzan even need something like battle.
Webeven human kokushibo is stronger than muzan because he has the second most powerful breathing style, he has the slayer mark, he can see the transparent world and he is the second most powerful demon slayer in the manga after yoriichi.
He is a lot stronger.