Highly doubtful that they can replicate haki, especially CoC and S-Mihawk would also lack a black blade.
I'm pretty sure that they on average gonna end up being very powerful high tiers, i.e. high high tiers ~ YC1lvl. With the right DF S-Mihawk might even end up being a low top tier, but there's where I see him at max right now.
Seraphims probably fullfill their purpose if they are strong high tiers "already", which is very incredible by itself.
I'm pretty sure that they on average gonna end up being very powerful high tiers, i.e. high high tiers ~ YC1lvl. With the right DF S-Mihawk might even end up being a low top tier, but there's where I see him at max right now.
Seraphims probably fullfill their purpose if they are strong high tiers "already", which is very incredible by itself.