The IS-2 gun was very accurate at the level of the King Tiger's gun, the IS-2 had good optics and its only problem is a long reload. One shot to the front was enough to scatter the Tiger. The pictures show who the tanks were at the seams, and this was common for the King Tiger.
It will never be as accurate as an higher velocity shell, which happens to be lighter.
And regarding optics king tiger had a x5 magnification (zeiss optics), one of the few tanks to offer it as opposed to x2/3 magnification.
And even if both misses the first shot, king tiger can fire 4 shells before is2 has finished reloading.
King tiger could penetrate is2 at 3000 ms with 40/43, 2500ms at 39/43, FAR MORE than the range at which an is2 could accurately hit with a low velocity gun.
But hey, there's a reason if only high velocity guns are used post ww2.