Well, we're talking about WW2 tech here, and it's going to be more accurate and a greater rate of fire, plus Nazi Germany's industrial capacity to produce a huge amount of shells. It is a ultra long range artillery gun. It won't win the whole thing by itself, but it will force the enemy to spread out in order to avoid a successful hit, because a succeful hit on a concentrated "Schwerpunkt" would result in massive casualties. A shell like that hitting London would level a whole bloc, and without endangering the Luftwaffe. A weapon that cannot be stopped, with no way of prevention, would result in chaos on the populace. If a shell smashed into a factory, it would be out of commission for a few months. It's meant to softenup the target. A series of brit forts on the coast could fall in less than a day to become useless if under constant barrage by a good dozen of those guns, allowing a invasion force to land and not face an enemy entrenched in fortifications, but weary from bombardment that lasted the whole night and stuck in rubble, even if reinfroced, without having the opportunity to entrench properly due to the arty, it would be inadequate to face the invasion force. It would be a D-Day without half of the casualties for Germany. The Allies couldn't fully concentrate an invasion force in an area, but would be forced to spread out in order to avoid massive casualties. It would soften and slow down any invasion considerably, leaving a mop up force to clean up the demoralised and weary enemy.
