From Gladiat's Sieg Skill Guide topic:
Despite it only showing the stage damages, it also gives a rough estimate of what your skill would do in PvP Balance if it's maxed as well. For example, whirling sword is about 450 per hit, maxed. In balance, the damage isn't that far off from 450 either.
That means Whirlpool should do ~300 per hit in PvP when maxed, not including decay.