not sure if it applies or not but from my testing as a water main , darui does more dmg than haku and mei does more dmg than darui in 9 tailes. so from all water boosting barrier ninjas i own mei seems to help me get highest dmg. quite possibly because that 25 % dmg boost works ? but cannot say for sure. possibly because she has better ninjutsu growths and all her atacks are elemental based ?