I think the balance comes from the perked vehicle being able to die - and the subsquent loss of precious perk points. While there are obviously those don't care about wasting perks because they have 24000 of each, they're obviously the minority players in the game.
Can a JS-2 be killed by a Panzer IV with a 75mm, an 85mm, or a Sherman's 17lber? Certainly. Much the same way (historically) that 75mm Shermans had to gang up on Panthers and Tigers to kill them. There's a well documented battle where a single T-34/85 with both standard HP and HVAP rounds killed 3 King Tigers from a concealed position.
No tank was impervious to anti-tank fire. Obviously, hitting a Panzer IV with a 122mm gun is going to be, should be, and historically almost always was, a one-shot kill. But tracking a JS-2 or King Tiger, or getting a flank or rear glacis shot, were usually enough to kill them, or at least immobilize them.
I think the perk system would have to be revamped for GVs, especially with the addition of tanks like the Panther, Jagdpanther, etc., that have decided advantages in armor protection and penetration. A good balance can be found by adding vehicles with equal firepower and less armor for fewer perks. They make up for inferior armor with lower visibility and good maneuverability. We obviously don't have to balance a JS-2 with a King Tiger, because a player could simply match another JS-2. The balance would come from having the ability to up a low to medium perk vehicle that was capable of killing the high-perk tank. If you take survivability out of the equation, and give that low-perk GV a low profile, you have a semblence of balance. Here's a list of possibilities, just thinking LW at the moment, but obviously, some of these apply to mid war well. Early war would require a new list of vehicles that we currently lack, Churchills, Matildas, Crusaders, Lee/Grant, Stuart, etc.
High Perk - King Tiger, JS-1/2, Jagdpanther, SU-100, Pershing, Panther, JSU-152
Mid-perk - Jagdpanzer IV/L70, M36, Su-85, SU-122, M18
No Perk - Su-76, Nashorn, Jagdpanzer IV/L48, Stug III, Hetzer, M10, M24, Marder III
In most cases, any of these non-perked tanks could kill a medium perk gv. Maybe even a high-perk GV if they're lucky or gang up on a player. Obviously, the medium-perked GVs could kill a higher perked GV. The trade off comes with survivability.