User blog comment:Eles99/Study of Shelling Critical Hit/@comment-114.79.55.55-20140508175126/@comment-24878656-20140508233146

Chinese admiral already tested this back in December. Some used Nagato and some used Ise-class on FBB (90 health, 99 armor) over a couple of thousand sorties. No conclusion is reached because no one knows if Type-91 is activated equipment or if it just adds armor reduction as a flat rate. However, what is visibly observed is that there is a decrease in marginal damage and a substantial increase in damage capacity, but only on high armor, high HP targets.

By the way, the confounding variable here is that your accuracy isn't accuracy either. If your firepower isn't high enough to pierce enemy armor, the damage'll also display as MISS. Furthermore, there is a heavy debate within the community regarding exactly what crit is. Without knowing if crit is actually crit or just overkill damage, and without knowing if crit works differently on each class of ships, I think it'll be hard for you to establish further information.

FYI, for what it's worth. The general wisdom in that particular community is that Type 91 is only relevant at firepower >130. The JP community <3s Kongou class too much to actually bother with Type 91.