It already has Elefant like range, except it can actually kill ATG and infantry whereas elefant cannot. Comparing it to Elefant, I think I prefer the ISU-152 since it can make an enemy pay no matter what their force composition is. 200 penetration at far range doesn't feel quite right for a 152mm cannon so moving it up a little bit would be fine. But it shouldn't be as good as an Elefant at AT becuase it is a hybrid assault gun, not a tank destroyer as OP stated.
If you could choose between a T-34 and a Sherman, what would you go? Most people would say Sherman even though it is worse than a T-34 in fighting tanks. However it is specialized against infantry while the T-34 is still terrible against tanks . ISU - Elefant relationship is similiar. The ISU can engage tanks and infantry alike, but it doesn't perform excellent in any of those tasks. An Elefant however negates armor gameplay which allows the player to have other anti-infantry dedicated units which are usually countered by armor.
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