Tera: The damage is only higher when Spirit of Bloodlust is up and it does only last 15 seconds, higher damage is possible by a Mystic. All of the data points we used to determine the Critical Hit multiplier shows that the base damage between Priests and Mystics is identical; Mystics just have an extra skill that deals damage and a self-buff that's later surpassed by the Priest self-buff for +Attack.
That said, Essence Leech is very strong against single targets as shown in those same vids your quote from Yunesakura demonstrates. They hit much, much harder than the main attack in the range of 5-7x harder. But yes, it does have to be channeled, so you can't move while it's hitting. That is also the primary way you return MP(tera gold) to yourself or to your party, so it's a core ability used quite often.
Also, there is predecent for a support character to have a 15 second buff they have to keep recasting, unfortunately - the Chanter from Aion. It had the two best short-term buffs in the game and they were literally spamming that buff in a party. As we mentioned, we think the durations for Mystics needs to be adjusted.