No guys you are wrong. Actually you are thinking about the ISU-122 not the SU-122. These vehicles used totally different weapon designs, only the caliber was the same.
Here for comparision:
The "Tankhunter" ISU-122:

The "Howizer" SU-122:

As you can see, the SU-122 is a mobile howizer, thats why replacing the KV-2 makes sense. Maybe it would have more range and would act like a priest but therefore it would have less amour which would be a good compromise.