You can use sponsor debt for pretty much anything, including backlash. Usually, you'll take 1 point of debt for 2 shifts, just like Fate points, but depending on your group, 1 point of debt could offset the entire backlash as well.
As for self sponsored, I've always seen this as an "if all you have is a hammer" sort of situation. Self sponsored doesn't really mean you are lending yourself power, it's just a mechanical way to describe someone who is very specialized in one area of magic. Try to look a bit closer at how some of your friends might approach the same problem from different angles, depending on what kind of job they are doing, and you'll know what I mean.
So paying off debt in that regard would mean that you are set in your ways. Sponsored pyromancy would make you volatile, prone to aggression, burn now ask questions later, that kind of stuff. Even if a calm or secure approach, or even a retreat might be a far better option. But because you are so set in your ways with your highly specialized magic, the way you use it is just the natural way to solve problems.
So if you are self sponsored in wards, you'll always be prone to taking the safe route, turtle up and wait first, and not act unless you are absolutely have to.
Self sponsored crafting will make you need to channel your magic through a well prepared item and fail when you have to use raw magic.
All in all, it's a more powerful version of the "Blind Spot" Box (YS179) to me.