Please be aware, a tank does not just possess high HP, they possess the following:
1) High Defense
2) High Parry (in PLD case, high block rate as well)
3) Multiple Mitigation CDs
4) High HP
As a "Tank WHM", you possess the following:
1) Moderate HP
2) Stone Skin (easily interrupted)
So no, you will never be able to tank anything unless you over gear them by an extremely wide margin (in this case, I rather you ask a Dragoon/Monk to tank, MOAR DPS).
Now, on to "Survivability", you mentioned how you can save the day in Ifrit HM after a botched nail phase etc etc, or some may even think, "I can eat two WotL without dying now, higher HP is so worth it!!".
There's one similarity between the two situation mentioned above, which is FAILURE, your team failed to burn the nails down in time, you have failed to dodge WotL properly. So IMHO, having higher HP means you expect to fail, and in your expectation to fail, you have gimped yourself during normal play (lower heal values than usual).
Instead of planning to counter failure with higher HP, it is better to prevent failure in the 1st place, thus I do not see any tangible advantage of having a higher than normal HP pool.
