Nicely reasoned OP but I think there are a few things you're missing. I play 2v2s so I can't really speak on the 1v1 front but in my experience the Panther is an awesome tank. The OP describes how the Panther matches up against the SU-85. Same damage and penetration with about half the rate of fire but the big difference here is the Panther is fighting weakly armored Soviet tanks with no turrets while the SU-85 has to match up against the frontal armor of heavy German tanks. That may sound obvious but its a really big deal.
- SU-85 penetration: 170 vs Panther armor: 270
- Panther penetration: 170 vs SU-85 armor: 180
In comparison, the Panther has a better Penetration vs frontal armor match up. Combine that with the Panther's higher health and it's pretty obvious that the Panther beats the SU-85 handily when engaged in a frontal slugfest.
Time to Kill @ range 20:
- Panther shooting SU-85: 28 seconds
- SU-85 shooting Panther: 42 seconds
But, because of the far superior maneuverability of the Panther (better acceleration, better top speed, blitz, turret) over the SU-85 (focus sight slowness) the Panther should be able to attain a flank against the SU-85 and get hits on it's rear armor which changes the numbers drastically.
- SU-85 penetration: 170 vs Panther armor: 270
- Panther penetration: 170 vs SU-85 armor (rear): 80
And of course, once the Panther gets close and forces the SU-85 to rotate, the SUs rate of fire will go down significantly even to the point of not being able to fire. That turret vs no turret match-up is a big deal.
Those time to kill numbers hold pretty steady throughout the range brackets except for range 50 where the Panther's time to kill edge increases sharply (up to a 19 second advantage) and then at range 60 where the SU-85 finally sees it's first advantage, outside of the Panther's gun range. Even so, it still takes a full 63 seconds for the SU-85 to get the job done.
With all that said, units don't fight in a vacuum. There's a whole battle raging out there right? Well, beyond the tank destroyer vs tank destroyer match-up there is another really important distinction which should be pointed out. The SU-85s biggest weakness are PGs with schreks. German Infantry based AT is a serious threat to the SU-85 and can kill it in just a couple of salvos. The Panther on the other hand has no trouble with Soviet infantry based AT. Guards' DPS rifles (doctrinal only) are pretty weak and the Panther has a good chance of resisting AT grenades. So, while the Panther can't do much against Soviet infantry, the reverse is also true. The Soviet infantry AT arsenal doesn't have anything that can directly harm the Panther.
Now, is the cost just right? I don't know. I think the jury is still out on that. I personally need to see more games before I can make that determination. But I don't think anyone can argue with the unit's performance. It's a beast.
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Some Relic dude read this and realized that he knows less about the balance than the community. =)