The power is ensured by the concave blade going more or less downwind. I don’t see how the Cp could be above 0.15. But in the second example I deduced a Cp of 0.3: I have no explain of a so high Cp for a Savonius rotor.
Moreover Savonius rotors using kite technics could have a Cp still lower.
If theoretical studies are not sufficient because being divergent, some experiment would be desirable.