Is2 fired 2 rounds per minute, and had around 160mm of penetration. They would not penetrate king tigers frontally. Only due to poor quality metals did they fracture the armor plating. Tiger 2s were better than is2s.
It's guys like this that really soured these forums for me, trying to bring realism into a game, only to be completely wrong. IS-2s in WW2 loaded pure HE when firing at tanks. The spalling and raw force from a 122mm HE round was all that was needed to knock out german tanks
If the Tiger 2 wasn't getting bombed, it was breaking down or running out of fuel. The gun was good though.