The bit about it reducing toughness and (maybe) ignoring catches is pretty significant, in my opinion. Consider that some creatures that might be completely imune to magic might not be imune to magic+soulfire. Also, the reduction of toughness is rather nice, too. Consider, say, a vampire that has four physical stress boxes, plus inhuman toughness that gives him armor:1 plus two more stress boxes. Without soulfire, you'd have to give enough stress to get into that non-existent seventh box, which might take four 5-stress hits, for example (before subtracting armor). With soulfire, a single 5-stress hit would be enough to force a consequence, because the armor and those two bonus boxes don't exist for purposes of marking off the damage. Compare this to Hellfire, which 'merely' adds one to the power of the spell, which translates to +1 stress. (But only if you can control the extra power; otherwise it translates to +1 backlash!)
In additional, all sponsored magic grants one additional ability that you didn't mention: the ability to use debt to invoke aspects. I think this is probably very powerful for spellcasters, as they are generally very low refresh, since it means that you can pump that extra bit of contoll into your spell even after exhausting all of your Fate (or before gaining Fate through compels), thus avoiding what otherwise might be a missed spell, or a spell that backlashes in an unfortunate way. You pay for it later through debt compels, but Soulfire's sponsor is likely a bit less demanding than some of the other sponsors...