Does this skill really give a damage boost?

When I tested for the damage multiplier for rank F I found I wasn't causing anymore damage than normal with magic or melee.

Can anyone give confirmation that it gives any damage boost at higher ranks?

If not, the damage multiplier should probably be removed.

If you were the one using the skill, you wouldn't be able to attack them.

It actually gives a damage penalty until the higher ranks, where it gives a boost.

I was using two computers and having the alt attack it while I played music, with 100 max I was hitting an observed max of 99~ and critical hitting for 249~ (this is with R1 critical hit). The mob used had 1 def 0 protect (Black Direwolf Cub).

As for magic my alt was causing a constant 233 damage both before and during the song with lightning bolt.