I would still be keen on seeing real life performances reflecting the units. The ISU-152 already deals big damage, like the real thing. Now it´s time for the reload to also reflect historic values. Even if the reload was doubled to 20 seconds, the clumsy ISU would fire 3x faster than the real thing. So the realism argument really doesn´t help you.
But a little reflection would be cool. At least give it a 15-20 second reload to somewhat reflect how it was used.
Would you like to simulate German mechanical failure as well? The quality of German materials and tanks were being reduced as the war went on. Seeing as the multiplayer is set in 1944 ish, should this be reflected as well? The moment you start balancing things based on real life, the moment balance goes out the door.