It's been proven? How? I'd always thought that the soul damage backlash to the caster was a myth, aimed at discouraging the casting of malign spells.
In order to prove otherwise, you'd need to know what the shape of the casing and the state of the soul was like before the spell was cast, what spell was cast, and then what the shape of the casing and the state of the soul was like afterwards. To say nothing of controlling for other variables, including the fact that people who cast these spells more frequently than the average wizard also have the spells cast on them more frequently than the average wizard.
And so apart from instances in which the casing actually ruptures -- which can happen in relation to both killing curses, and repeated casting of the Cruciatus -- it's difficult to even measure or diagnose strain. And that's before you try to attribute cause.
I'm sorry to say I don't know of any treatment that would. But if you can tell me where you read about this recent testing, I could look into it.