Why rearming the Mogami class with 8-inch guns was a mistake

Did the Imperial Japanese Navy make a mistake replacing the Mogami class's 6-inch guns with 8-inch weapons?

Yes; without strong radar, the 6-inch battery's volume of fire offered more practical combat value.

An 8-inch battery becomes compelling only when radar allows a navy to exploit its range in poor visibility. Without that sensing advantage, a faster-firing 6-inch battery can place more shells on the target under the conditions in which the cruiser is actually likely to fight. Those shells can produce a mission kill by wrecking radar, directors, and other equipment outside the armor belt even when they do not penetrate the citadel. Scharnhorst's loss of radar before Duke of York arrived illustrates how disabling exposed systems can decide a battle.