The comparative improvement in hp between Endurance stacking and mitigation stacking setups is all of 25% greater hp (24k with mitigation stacking and 30k with Endurance stacking), which will amount to all of ~30 additional hp sec (going from roughly 160 hp/sec to ~190 hp/sec).
The *only* time that this would be even remotely intelligent would be when you're taking part in a fight where the absolute entirety of damage coming at you is F/T based. The only fight like that is Soa. You could count SM Firebrand and Stormcaller as well if you don't swap, but, even then, you still take some amount of M/R damage from Firebrand during the shield phases. Even if you did count it, you'd be bringing a second set of gear for 2 fights that aren't even progression fights any more, which is insanely wasteful.
As such, HP stacking isn't useful no matter how you end up justifying it. You get all of the Endurance you need no matter what you do so there is no point in stacking it up.
Isn't the OP talking about Nightmare Mode EC??
It doesn't matter how much mitigation you have, you still die once your HP reaches zero, which is why 1.8k hp for 1.5% abs is a very good tradeoff.