The 'with Soul Stone restores 50 MP' brings up an interesting question that isn't really clear.
Since the spell costs 50 MP to begin with, does that mean that using the spell with a Soul Stone restores 50 MP IN TOTAL (as in, if he started with 100, cast the spell, and he now has 150), or does it mean that basically the spell is free (as in, he started with 100, the spell cost 50, and then restored 50, leaving him at 100).
In either case, I wonder if he could use a Soul Stone to cast it when out of mana?
We don't know if he can cast it on himself, though. So far he's only used it on Sun-Il. So with a Soul Stone he'd be able to restore someone else's mana/ki.