I think you're ignoring the on-use for the dreadguard relic. With the extra 350 power up 1/4 of the time, you have an average of 134 power from the DG relic. The WH relic gives a constant 113 power. Both of these are better than 66 WP and 27 crit.
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.