Short answer: yes, it is worth it. In fact, the Dread Guard proc heal relic is best in slot for shadow/assassin tanks right now, with other relics trailing fairly far behind. I haven't run the numbers on the new PvP relics yet, but I suspect BiS is now EWH defense paired with DG proc heal.
Quick reminder: bosses in TOR hit like a wet noodle. 500 points of healing is actually quite a bit.
my shadow is still using the campaign relic for now because of the higher endurance along with the new dread guard absorb proc
i don't have the extra daily comms on him right now to upgrade the healing relic, but i'm not sure if i really want to.
I know these may not be the BiS's but I run the Matrix Cube and the Campaign Relic of Shielded Amplification (on hit proc 405 absorb) I've tanked EC HM, TFB SM (no HM yet because he's not my main) seems to be descent enough combo and I don't feel I'm too squishy (I also have a PT and Jugg tank and all 3 differ as they should).
on your assassin tank you're using the matrix cube? it's horrible. you'd be much better off with a champion def relic, and you can buy it for 11k