no. In the books, it specifically states that Soulfire works by using your own soul to supercharge your spells.
But what it really comes down to is, the books specifically state that its a path to boosting your magic, not a sponsorship situation. Add to that all the times (Both in the novels and RPG books) that they allude to and illustrate that "Gods" plan is more concerned with human freewill than the triumph of good over evil, and it just seems to me that the sponsored magic system would need to be amended for Soulfire.
(emphasis mine)
I'm assuming that you mean in the novels, and not in Your Story or Our World. There is a difference between how soulfire is presented in the novels and how it is presented in the RPG. The RPG clearly labels it as sponsorship, states that all spells are subject to sponsor's approval, and states that "we" (meaning, "billy's" understanding of the subject) don't know much about the sponsor's agenda. It states that the sponsor has a potentially gentler agenda [when compared to that of hellfire or other sponsors], and suggests that it may be stricter in terms of debt allowance due to this.
I'm fairly certain that this change (to clear sponsorship, rather than "just an improvement on your power") was done to keep soulfire in line with the sponsorship rules, and to provide a limitation on its use...otherwise, you're getting full thaumaturgy and channelling fire PLUS toughness reduction on
all creatures for the same cost as normal thaumaturgy an channelling fire. Without limitation, it's just out of balance, and every caster should take it instead of normal wizarding.
In the RPG, at least, you AREN'T free to use soulfire however you want. It has to line up with the mysterious sponsor's agenda. Thus, the need for a thread like this to help us get some ideas on what that agenda should be.
Even in the books, redemption from evil is always key... as we've said, Michael has to offer the Denarian's host a chance to give up the coin before he can just slay them. He's free, however, to just wail on black court vampires, demons and the like... or so it seems. The nature of the Swords of the Cross themselves (destroyed if used to spill innocent blood; having the form of a weapon) implies that they are to defend innocents from evil, even using lethal force. So while I totally buy that free will is an important component here, I'm not willing to down-play good vs evil. Remember, Harry got soulfire after giving up hellfire by helping to redeem the shade of a demon from his own mind. Redemption seems more key than free will.