So I did a test for Sakuragi where 1 item AC = 6.5HP for CH, i.e. a warrior with 5000hp/1000AC and a warrior with 5650hp/900AC will require complete heals at about the same rate.
Let's assume you're fighting unslowed mobs which put out 25 dps roughly, and that you have 4000HP with shaman buffs as a paladin who isn't max level. 11AC ~= 65hp so you'll be CHed 1.5% less often or you'll take 1.5% less damage. 1.5% of 25 dps is .375 dps, or about 2-2.5 damage per tick. Meanwhile the BP is healing you 5/tick. Clear win.
Now let's say you magically discover Hagglebaron's stash and get a fungi. 15 hp/tick is 2.5hp/second which is 10% less damage. That would require 400 hp equivalent or about 60 raw AC. Which means that even a cobalt BP (+25AC, +140HP) is clearly worse than a fungi tunic for group tanking.
Of course, the more damage/second you are taking the higher the impact of AC. So if you are taking 150 dps from say the Overking than the fungi is absorbing 1/60 of your damage, 4000/60 = 65HP equivalent, and suddenly your BP is almost identical in effectiveness.
Anyway just some back of the envelope calculations. I haven't really done good tests on the effectiveness of AC at high levels, probably because my iksar warrior can't even get about 1k without a shield
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