is he stronger? no , can he beat him in a fight? yes
He simply has a genjutsu hashirama can't counter since he not only doesn't know about shishui, he doesn't know about it..
Exactly, But it goes deeper, It's not just an idea anymore..
with the idea planting, the person still has a choice to do it or not.. but the idea is there... Shishui is almost complete manipulation.. he can implant memories of him being your friend all your life, and you will believe it, because you think it happened..
It's like that dude from bleach, tsuchimama or w.e