Like you said number that you can see beside every item on wow-heroes is just a Blizz Ilevel ,that isn't score for that specific item .
Enchants don't have Ilelvel , level that you can see for them is just a level when enchanter can lvl up (skil up) on them.
Your survivability will be more than enough if you have proper items for your level with +10 stats . Healers and tanks in PvE are there to think about you (not knowing tactics ,or not watching on your aggro is only your problem and that is another story where 275 health can not save you).
Enchant Chest - Super Health is high valuable enchant for all tanking classes.
None of the theorycrafting websites we've seen suggests Super Health as the best/or over Super/Powerfull Stats. Also all high end raiding shadow priests use only Powerfull Stats .
In specific fights where you need more hp than usual most people use different/stamina gear, or at least a trinket like Essence of Gossamer