Maybe a little off topic, but do you Tanks ever carry around multiple sets of Tank Gear? (or at least pieces that you swap in and out, maybe a different Relic, change a chest piece etc...)
I change my heal procing relic to a shield procing relic on kephess nim, then change it back to the heal procing relic after the trandoshian phase. I have noticed it helps out the healers a bit more for that phase but overall probably would be fine if i didnt do it.
When it comes to question of mitgation vs HP, i usually work on a balance between the two. I use only letter mods, but will use a mix of high and low endurance enhancements, and will never use end augments. I usually aim to get the most endurance + mitigation stat, over willpower. This leaves with ~29k hp on my darksin, in the worst situation possible (tanking NiM Kephess while having the bleed (the other tank went splat because they a pure hp build)) I was able to survive with minimal stress on my healers (was a Merc and OP healer) without the need of CD's. I feel that if i was full mitigation or full hp build, i wouldnt of been able to survive in such a situation due to the fact that the a large amount of the damage i was receiving was not being mitigated.