You are bad at math.
100% boss damage with the decrease to the final damage is:
400% damage total = small buff to solo damage
500% damage total = about neutral
600% damage total = less damage (small nerf) to solo damage
This ignores the vanquisher charm buff, which is 10% final damage to the spell itself.
If you go like "Well as long as you have less than 539% damage in total it's still a buff", then that's not a buff; it's setting a limit to Kanna's progression.
And 10% increase, sure. 10% to the skill that takes about 10% of whole BA measurement (given that you can use this skill fully) is a buff to compensate the nerf, right?
400% damage total should be treated like 95% ied and you want to avoid going higher than this as best as you can, if you are hitting this number then stat and attack option for more damage will be better. (outside of crit damage which is a completely separate system)