Actually, the dread guard relic lasts 30 seconds, on a 2 minute cooldown, so it's up 1/4 of the time. In that you're right. But 1/4 of 350 is only 87.5. WH relics top that. Heck, Champion relics top that with 93 constant power.
Also, pertaining to the matrix cube, based on mods, crit and power are exchangable on a 1:1 ratio in most cases, and 66 WP would be equivalent of approximately 72 or 73 power, plus the 27 crit would be 100. Still beaten by Battlemaster relics.
The dread gaurd relic also gives passive power. It is better than war hero.