IS-2 would be an interesting though largly useless perk tank, it had a miserably slow rate of fire and only 28 rounds of ammo in 2 pieces. The D25T 122mm gun had great HE cabality due to its 25kg shell, but anti-armour capability was only roughly equal to Panthers 75mm Kwk42/L70, however Panther also had vastly superior optics which made shooting at range much more accurate. That said Stalin had thick frontal and side armour and a good shilloute. TigerII would be a much more dangerous perk tank than Stalin. The 88/L71 had much better penetration, accuracy, ROF, and TigerII carried some 70-80 rounds of ammo. Frontal armour is superior, side armour slightly better, and shiloutte inferior compared to Stalin.
thanks GRUNHERZ