Big guns are horrible in the game. In fact, the smaller guns you have, the better you fare.
(Relatively) small gunned ships tend to have more guns, more accurate, has better reload times, has better radius damage (365mm Japanese SAP rounds).
It is like using a bolt action rifle against using a MG42 (except bolt action rifles are actually accurate).
Mutsu, 8x 410mm
Yamashiro, 12x 365mm
Realistically, shells does similar damage when you penetrate a enemy anyway, black out the module. Because of how inaccurate the guns are in game, combined with the non existent underwater shell diving, small guns actually give you a better chance to damage the enemy, and more of his modules.
This is esepcially relevant when many ships are well armored in game, which demand a high volume of fire, in which some shells will make it into the weakspot. This is something the Scharnoest very much excels in (a hidden gem of it).
The ONLY benefit of big guns is 6km duels where you can accurately target the enemy’s modules.