The SK's at evilgamer.net have it all down.
There is a softcap, but not as most think it is.
Softcap is 1300.
What that means, is anything up to 1300 is working at 100% effectiveness.
Anything over it, is at 66%.
Shield ac ignores it. So an aug'd shield at 140ac , will raise you to 1440 at 100%.
At the moment, there is no point at which ac stops working, like back in velious.
A really good ratio for choosing upgrades, is
1 shielding = 10 avoid = 15ac = 90hp (until caps of 35 shield and 100 avoid)
I follow a 1ac/6hp if there are other mods to consider, otherwise I go 1/7.
You only "need" enough hp to survive being one-rounded by the content you are tanking. AC can fill in the rest, which then lowers your hp needs.
4500 ac is my goal, but there is a 5kac sk out there.
One example of how auging ac will help over auging hp.
3 tanks. 2 wars and an sk.
wars are 25k and 23k hp. SK is 18k. But the wars have 3200-3500ish ac. SK has 4300.
All have divine intervention.
The 23k tanks. Defensive drops, and his health goes up and down so fast, he dies before DI can land again.
The 25k tanks. DI fires 7 times before the mob dies.
However, the SK was able to tank the same mob, from 100 to 0 , with less healers and less dps, meaning a longer fight, and his DI only fires once.
That's with "at least" a 5k hp difference.
HP will always come with the gear. It's given away like water, because it's the ac that's trivializing encounters.
There is just no really justifiable reason to take 1500-2k in hp augs over 300-400 in ac ones you could have.
You know who that sk is btw. Ask him about the fight.