Best in slot for tanks is generally considered to be the gear which maximizes survivability. The proc heal relic does exactly that. You can go with the activated power relic if you want, but that's not best in slot. It's an off-beat, albeit mostly viable gear choice, but not best in slot. What you are suggesting is analogous to wearing a piece of DPS gear to improve threat, rather than sticking with all tanking mods.
Threat is a non-issue for assassin/shadow tanks. Go with the proc heal. It contributes substantially to overall survivability.
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).
I'm wondering with this talk of the healing proc relic what about using that with the DG Relic of Shielded Amplification?
Now this isn't a pun at all Sin Tanks, I normally play either tank or healer mostly healer (I have all 3 healers all 3 tanks full Campaign or better gear and generally think I do good at it). I healed a Sin Tank that had 2 proc heal relics (wasnt the best choice to use 2 but he he said he was working to get a different 2nd). Anyways his gear didn't seem terrible but he got hit like a truck compared to what I feel while on my Sin with his relic setup. So again I'm just curious why the healing route vs the mitigation route being touted as BiS and some numbers to go with. Please not attacking just looking to share info for success!