Just reread and what struck me was that semi-divine shapeshifter's reaction to soul fire. It clearly bypassed at least some of it defenses. That, and by how rare that thing thought soulfire was. From that reaction I'm thinking you need to get it from an old god or archangel.
That passage really seems to put soulfire in a class by itself and probably changed someone opinion of Dresden, because if he had soulfire then he couldn't be all that bad.
I'm thinking that we will learn more about that power when Ghost Story comes out.
Richard