Unleash is 150 potency, and Stalwart Soul is 160 potency.

On two targets, that is 300 potency plus 320 potency for a total potency of 620; this results in a potency per GCD of 310.

Hard Slash is 200 potency, Siphon Strike is 300 potency, and Souleater is 400 potency for a total potency of 900; this results in a potency per GCD of 300.

Unleash and Stalwart Souls are spells unaffected by skill speed, so they lose some potency per second, but they also generate MP and Blood faster than Souleater combo.

You are embarrassing yourself.