Sure, you could hit a target that far away and I wouldn't stand out in the open if some one was shooting any rifle at me from 800 yards. Still, the best accessory for engaging targets at really long ranges with an SKS is a good pair of shoes. Walk 3/4 of the way there and then shoot it.
The round was developed toward the end of WWII. I'll spare everyone the story they already know about the 8mm Kurz and all that. The long and short of it is that during WWII they realized that most rifle fighting happens at 400 yards or less. 800, 1000 yard shots do happen, but the overwealming majority of the action happens from arms reach to 400. That lead to the development of rifles and rounds that are designed around that central theme. Where the old paradigm was to give soldiers rifles capable of hitting anything that can be seen by a young man in his prime with the best eye sight he'll ever have (30-06 battle rifles, etc) the new is to have rifles that do what is most likely and carry a bunch more ammo.
The SKS was designed at that time and was one of the first fielded in that genra. Not the first, but it is definitely a first generation intermediate cartridge rifle and it does it's thing within 400 yards. That's what The Boss ordered and that's what Simonov delivered.