You seem to not know much about anything in regards to how these vehicles actually performed. Let me be a little clearer. The is2 could not penetrate the king tiger frontally, in a few situations where king tigers with poor quality alloys were used the frontal plate of a king tiger would eventually crack after a few hits. This has nothing to do with actual intended performance or faulty design. Its more to do with indirect means such as low resources for the german war effort. Do you also blame tanks for the countries inability to provide fuel for it as well?
Is2s did not penetrate king tiger tanks under normal circumstances. For gods sake learn a little something about ww2 before you try to argue balance in terms of historical nature.
The king tiger KWK 43 couldn't penetrate the IS-2 front armor under "normal" circumstances either. Infact on paper IS-2 was completely immune from the front, just like the king tiger.
However this really was not the case in reality.
IS-2 is more on par with the KT than the tiger if we go IRL. However, in game it must be balanced with the tiger I, since they both cost the same.
Of course we could make the IS-2 like a KT and make it cost like 320 fuel or something.. But i dont think thats a good idea.