Took a few of these 'rare' situations for my healers to trust me and realize that while spiky, when needed, my shadow will stay alive beautifully.
That's not really due entirely to your higher hp though. Pretty much any Shadow can do the same, regardless of gear preferences as long as they know what they're doing. At 31k hp v. 26k, you're getting ~3800 hp out of Battle Readiness when it's providing 3300 to someone with lower hp. That's all of 500 hp extra every 2 minutes, which isn't really going to amount to much and isn't really going to save your life (unless you regularly get precision killed by non-instagib mechanics which would just be strange).
You're making a *lot* of claims about how amazing higher hp is when those claims have little to nothing to do with your actual higher hp being made useful. If your TfB tanking situation, any Shadow tank could have done that just as well: if you're already low, you blow a CD; if you're not you just tank it because it's not like the tentacles hit especially hard. A high mitigation tank would simply take less damage and see the same result. I know because I do this regularly. I even do it on my other tanks because every tank can handle not being healed for a reasonable period of time, more if they burn a CD.
The *only* way that higher hp is going to have been the defining factor in your survivability is if you're hit by an attack or attacks that deal in excess of 26k damage (or whatever arbitrary hp assignment you want to use as the "mitigation stacking" hp) in a time frame before your healers are able to react. Nothing in the game hits this hard at this time. In fact, even if you plan for the absolute worst case of a chain of attacks getting through defense and shield, you have to arrive at something like 9 consecutive unmitigated heavy hitting ops boss attacks to actually kill a tank from full health. Nothing hits that hard on a regular, fast enough basis to actually make this a concern; you *can* plan for it, but it's like planning for a volcanic eruption in Kansas or a nuclear strike on the continental US: it's possible, but it's just not really likely to happen (especially since that's 9 consecutive unmitigated attacks without receiving any healing; you've got the have the superstorm of bad healer *and* bad luck *and* no CDs).