I don't see a lot of difference between this and what's in the book. In YS, it already counts as 'holy' and 'fire' for free, and also counts bypasses one level of Toughness all of the time. In addition, you still have the usual option of spending Fate to further boost it or to accept debt to further boost it.
With regards to the hunger rules ... well, the idea of burning off soul and needing to slowly replenish it does sound a lot like hunger. Or at least the way hunger ought to work. The rules in DFRPG seem very quirky to me at best, though, so applying them to Soulfire doesn't appeal to me. After all, you'd only need to not use Soulfire for one scene to get back that Soul you burned, right? In any case, as stated it would be pretty trivial for a Wizard with a decent Discipline to make use of 1-2 free Fate worth of Debt per scene with no ill effects -- and even if you got a mild consequence, it would go away in a scene -- whereas with the Debt rules each Fate borrowed gets repaid in kind.
I think that the Debt rules are pretty decent, though it might make sense to tweak back the way repayment of Debt works. For example, it might make sense that the Debt is not repayed via compels at all ... instead, any new Fate you earn is automatically applied to buy down the debt. Harry doesn't seem to be forced to do much of anything to 'pay' for his use of Soulfire; he just is weakened for a while (which translates here to taking longer to build up more Fate).