Imo one could try the following:
1) Add an extra tech for super heavies (and maybe all doctrinal units) to help adjust cost and timing.
2) reduce their all around performance creating certain areas that they perform inferior:
For instance:
Tiger could have relatively good mobility, good performance vs hard targets, lower vs soft targets
IS-2 could be very durable, good performance vs soft targets, lower vs hard targets.
and so on.
