Yes, the HP vs mitigation thing entirely depends on how hard you are getting hit. Same with any shield really, and Shield+Mitigation will always combine more efficiently, (IB+Defiance, TBN+Grit) and why shelltron has stronger mitigation as pld lacks HP synergy. But (imo) the reason why war has so much 'ease' sliding in and out of tank stance is because their cornerstone mitigation is locked behind defiance and a GCD itself. Think of it this way: War/Pld have access to their primary mitigation all the time, but additional mitigation in tank stance is hard to access. War have easy access to tank stance but hard to access primary mitigation.
War is MUCH easier to use tank stance on, but that is because IB is very hard to access so they smooth out tank stance and reduce penalties (unchained, OGCD, etc). As long as IB is a GCD and behind defiance and TBN/Shell is OGCD and freely accessable, the tanks will not be able to work in the same way. This is not to say that Pld/War should not ease the costs (I personally would rather they leave them GCDs but reduce/remove MP cost and make sword oath like grit, click it off not another GCD on the backend). But with TBN/SHell on OGCD and stance free, I don't think the stance also needs to be OGCD without even more dramatic changes to all 3 tanks which would end up homogenizing them even more. Though bringing IB out of defiance would allow SE to put war defensive actions on similar timers and remove the 'extra' CD war's have, make tank stances all similar, etc. But that is a question of is homogenization for balance better than awkward balance but distinctive jobs. /shrug. If we are under the assumption that homogenization is 'bad' then I cant get behind making drk/pld tank stance as easy/forgiving as war's because war trades awkward IB for EZ stance and Pld/Drk have EZ TBN/Shell but awkward stances. (Also drk needs a snap enmity move to match scorn/onslaught so that tank stance isn't needed for add grabs just like Pld/War and not rework tank stance around Drk missing a tool. I like Dark passenger for this).