I've decided not to run soulfire with sponsorship debt, but rather with a stress track determined by discipline. I chose discipline partly because conviction was already used for the mental stress track, but also because building your soul isn't done simply by how much/strongly you believe, but by the actions you put behind that belief. Bob didn't tell Harry to rebuild his soul by believing harder, but by doing nourishing things. Nourishing things aren't always easy to do or fun to do and that is much more the case when it comes to the higher level spiritual stuff, and in those cases it's about discipline.
1 soul stress to perform an evothaum spell if you allow evothaum with Soulfire, 1 stress per point of power or control. The stress track doesn't clear at the end of every fight though. You have to spend time and make rolls to clear stress. I figure 2 points of stress per declaration. Time frame for the declared action is negotiated between the player and the GM, but it should be fairly common sense. Extra declarations can be made for longer actions. For example, a whole day of meditating might clear your 1, 2, and 3 soul stress boxes, but dinner with a friend might only clear your 2 soul stress box.
Since soul stress boxes don't clear as quickly as other stress boxes, you only need to use those boxes when you perform evothaum, and when you're calling up power/control above what comes naturally to you. It works very similarly to debt, but reskinned. Debt to absorb stress, and debt to invoke aspects for free, but in this case, you're using a stress track you need to actively clear rather than debt and compels.
Feel free to critique.