Not really sure where exactly do you get these facts that soviet optics were inferior at all. Never heard about that IS-2 gun not fitting the turret either.
The IS-2 ( and I guess the USI-152 ) had two-piece-ammunition ( of which it could only carry about 28 ). You basically had to load the weapon twice, to shoot once.
Also,
as far as I know, to reload the gun it had to be lowered after every single shot for the loader to do his job; the turret didnt offer enough space for the reload mechanism I believe.
You could only load a fresh shell when the gun was in a certain position, ( which ment you had to take aim again after a reload ) -
I dont think IS-2 was really a Battle tank and more of a Break-through tank.
correct me if im wrong