Would the backfire cure the ailment, and then apply the extra bonus? If not then theres no reason to use it, you should just be -na'ing.

If it DOES remove the ailment, then IDK it seems pretty cool, but sounds more like a WHM ability to me. Also I think having abilities that rely on ppl getting enfeebled would be extremely limited, and they'd probably have to be overpowered to make them useful.