My memory failed me on this one, I thought the buff was for 20 seconds, but it is 30, which brings it more in line with the other relics:
+6.3% for 1/4 of the time is +1.575% worst case, or +1.89% in a 5 minute fight. By stacking with Inspiration, Zen, etc, it is probably over +2%, quite close to the others.
But wouldn't the constant power relics also benefit from Inspiration/Zen? (I don't know if the damage proc relics are affected by inspiration. You mentioned earlier that they aren't affected by Bonus Damage or Force Bonus Damage).