Depends from which patch. The Panther at vet 2 used to have around 320 armor and 960 HP for years. The was changed for one patch when they removed the 10% armor bonus completely and in the next patch then they moved the 160 HP bonus to vet 0 lowered, the armor and reintroduced the vet armor bonus.
Even so your claim than Panther has traded Armor for HP in vet and thus it is buffed vs vet 2 Su-85 is simply false.
Bottom line currently the vet 2 Panther is the weaker version when it comes to durability.
Although patch history is interesting, it also rather irrelevant. What is important is that SU-85 vs Panther relationship changes from vet 0 to vet 2 drastically for no apparent reason.
The SU-85 at vet 2 can damage even a Panther vet 3 with 100% chance which translates it vets faster and better than Panther.
When it comes to facing vetted Su-85/M36 the armor bonus of the Panther is quite pointless and the unit would benefit more from a different bonus.
Your data is correct, but I disagree with your conclusion.
You say if a SU85 pens a Panther with 100% chance then we need to buff the Panther's armor.
Since both units are tank destroyers, we could turn the whole argument around and say: a Panther pens the SU85 (actually all Allied TDs) with 100% chance. Now do we need to buff the SU85 armor? I don't think so.
The veterancy concept for Allied TDs is "high damage glass cannon". So the Allied TDs get a pen buff, also because Axis armor is quite heavy.
A Panther does not benefit from pen buffs that much as Allied armor is quite light, so Axis vet follows the concept of a heavy brawler.
Basically:
Axis armor buff does not help the tank against Allied TD, but against all other AT weapons
Allied pen buff does not help against medium, but against heavy armored tanks (Tigers, but also Panther).
We can discuss about whether the current amount of veterancy buffs is alright or if the base stats should be altered slightly, but this would be best discussed in another thread.
Back to topic:
This thread is about StuG and JPIV penetration (vs medium tanks), not about Panther vs SU85