I nerfed it to a regular 4+ Regen save. That was mostly because I was interested in trying to make a statline I could see GW actually creating for Mathilde, and there was no way they'd have a character come back from the dead more than once. And in 8th they dropped immunity to fire as a rule, and gave everyone who used to have it 2+ wards against fire instead. But yeah, I absolutely took the canon item and beefed it up a bit for Mathilde's Seed.
Funnily enough, even your nerfed version is stronger than the canonical Jade Griffon, Volkmar's magic item (5+ Regen Save). GW were very tame about the majority of the Empire's magic items, which makes sense considering they're the "low magic" faction (weapons like Ghal Maraz and Runefangs are outliers).
On another topic, I've noticed something about the current crop of P/Matriarchs in DL.
Namely, that many of them are frankly ridiculous.
Elspeth is obvious. You know, ashes of a dead god and all that. Dragomas has also been discussed extensively, he is certainly absurdly powerful by any metric. But I think it bears mentioning that the others are no slouches either.
Rereading Storm of Magic, it seems to me like Paranoth and Algard are really big deals. Paranoth is one of the few humans (and maybe the only currently living human?) with a Mythical Artifact in Storm of Magic. I mean, all things considered its effect isn't that ridiculous (it protects a pretty big area the size of an encampment, turning a forest/swamp/hill whatever into a "building"), but it's still considered a Mythic Artifact. And he has to be absurdly powerful to travel "every road in the Old World and Ind". In canon he expands his portfolio to all the roads between Naggarond and Cathay.
Algard is also not to be underestimated. Algard is one of the few mages to get a spotlight in the book with a scenario presented for his Towers of Screaming Death, which I should remind people are teleporting towers that "no longer reside fully within the mortal realm" and also they don't obey paltry rules like physics and gravity.
The whole "moving the sun around" thing he just casually threw out is apparently part of his M.O. Algard just casually breaks the rules of physics. It should also be noted that the name of his towers bear a startling similarities to a particular feature known as the "Portal of Screaming Death". As the name implies, you enter the portal and you die.
But hey, that's just four out of eight. Alric is very powerful, having maintained three terms, but he's pretty old and likely getting weaker. Reichtard is described as not a risk taker and we don't know much about him. Feldmann is more of a politics guy than a powerful caster. We don't know much about Stossel. Maybe things are balanced out as a result of that.
EDIT: I forgot to mention the very important fact that Paranoth's magic item is called "Paranoth's Piquet Fence". Crucial detail.