The frontal Armour of the IS2 is almost twice the pen value of the Panther's main gun. From the front, you'll penetrate roughly once in every two shots. What that means is, you'll statistically be able to pen from the front every 18 seconds (Assuming the round actually hits). Whereas you need something like 6 shots from a Panther to kill an IS2(18 * 6 = 108 or just under 2 minutes, completely ignoring the (actually quite large) chance of a miss), the IS2 only needs 3 shots on the Panther from the front, whereas it has a 75%-ish chance of penetrating.
Hmmm...
Panther:
Dmg: 160
Armor: 290/110
Hp: 800
Pen: 260/220
IS2:
Dmg: 160
Armor: 375/205
Hp: 1040
Pen: 250/190
First of all, IS2 needs 5 penetrating shots and Panther 6,5 (6*160dmg and something dealing 80dmg like a deflected faust)
Second: at max range it´s 58.6% vs 0% Well at 40 range it´s barely better vs 65%
Third: did we forgot the crappy speed and turret rotation of the IS2?
For barely similar cost i prefer 2 T85s, even without mark vehicle, rather than an IS2 to face a Panther. I wouldn´t want to dispatch an IS2 against a KT. Circlestrafing/blocking with 2 T85s is better.