Interesting thread, terrible grasp of control testing and statistics.
I'm going to summarize what I feel are the important questions. I'll also propose some ways of testing- I don't have time right now to do any testing myself, but may get around to it in the next few weeks.
#1. What is the order in which AC/Doge/Incorporeal/Concealment is checked?
As has been hashed into the ground, it's not actually relevant in which order things are checked, in terms of total damage mitigation. However, it is relevant in terms of perceived utility. If AC is indeed checked last, then we will see fewer "misses" than might be expected, and AC will be undervalued. Example: I run around with 25% dodge, 25% incorp, 50% displace and 120 AC in EE. Let's presuppose that 120 AC gives me 20% misses in EE content (I'd be happy with that, but I doubt it). If AC is checked first, in 100 swings I'd expect to see 20 misses- that's pretty noticeable as you're playing, and you associate that with AC having value. But if AC is checked last, in 100 swings I'd expect to see 100*(0.75*0.75*0.5)*0.2 = 5.6 misses. I might perceive this as being 1/4th as valuable as seeing 20 misses, but they're actually of equal value, as order of operation is irrelevant to overall mitigation. So, order doesn't actually matter, but it definitely does matter in the visual feedback we get for seeing AC as being worthwhile, and I'd like to see it confirmed. The OP's data and conclusions in this thread are neither conclusive nor rigorous.
How to test, without enormous sampling sizes: An easy way to test: get a toon with high AC (lets say >100), unequip all blur/incorp items, maximize dodge as much as possible. Step into normal Butchers Path. Record the results of a hundred hits (large sample size is not needed here). Let Y be the characters %dodge chance. If dodge is tested first, we expect to see Y dodges. If AC is tested first, we expect to see a much lower number: we have 100 AC in a lvl 2 quest, and there will be many many misses. If AC is checked first, we expect to see some number much smaller than Y, likely less than the order of Y/10. This test will order AC/dodge.
Next we need to order AC/concealment. As above, but use a setup with high AC, displacement, and minimal dodge/no incorp. Same logic applies.
AC/incorp may be ordered the exact same way. Setup with high incorp (25% shadowfade would be ideal), high AC, no concealment, minimal dodge.
If we don't care about the incorp/dodge/concealment ordering, and it doesn't seem that anyone does, we're done now. If we do care, then we need to setup a displacement/incorp scenario with minimal AC and dodge (here it would be good to use a highlevel quest to minimize misses due to AC). Subsequently we'd need a displacement/dodge scenario, with max dodge and displacement, and minimal AC and no incorp. That ought to complete the ordering (perhaps redundantly, based on the x/AC pairing results, we're trying to minimize sampling sizes not to minimize trials).
#2. Is high but attainable (let's say 150-200) AC effective in EE damage mitigation?
How to test:This is very easy to test as well. Bring a 160 AC toon into various EE quests with a healbot. Minimize the dodge chance, do not wear concealment or incorp. Let beasties swing at you, count the misses from 100 attacks. If the miss % is appreciable, it'd be nice to test versus several different monsters/areas, ie giants in EE Storm Horns, archers in EE storm horns, hobgoblin warlords in EE GH, to see what sort of heterogeneity there is in monster to-hit. Personally, I've seen very few misses to AC in EE content, but I tend to run with very high dodge/displace/incorp, so if AC is indeed checked last, that may be why. It sounds like others in the thread have used this testing method and found AC to be sorely lacking, but verification would be nice.
#3. I haven't seen this question explicitly, but I think it's relevant: does AC function differently in elite difficulty vs other difficulties?
I suspect that the actual to-hit formula has built-in biases depending on the difficulty. If we think that the monster to-hit formula actually is hit% = AB+10.5/AC*2, then the hypothetical result that 160 AC results in 20% misses for a mob means that mob has 246 tohit. That seems a little high, but it's possible that monster stats scale exponentially with CR. This one is a little bit harder to get at, but I have an idea. If we think that monster to-hit is just a factor of CR, then we might expect AC to be as effective in heroic elite High Road quests (~CR24 trash) as in epic normal (~CR24 trash), for the same quests and monsters. So: hop into heroic elite Stay at the Inn with something like 80 AC, no incorp/conceal/dodge (ideally would like to see ~50% hit rate), kill everything in the first spawn but the archers or the melee, and record 100 attacks. Recall out, reset, and reenter on epic normal, keep the same monsters alive, same procedure. If roughly the same number of misses due to AC are observed, then monster to-hit calculations are probably working the same across difficulties. If they aren't, then maybe we've hit on something about why 160 AC feels so lackluster in EE. NB: I chose the above example because I think the heroic elite/epic normal CRs ought to match up well. If they don't pick a different chain: either heroic elite/epic norm Wheloon prison or heroic elite/epic norm Storm Horns are potentially good candidates. Additionally, I am making the huge assumption that monster CR is the primary input of tohit, for a given monster type. Finally, I honestly don't even know what the current glancing blow implementation is, so some information here would be nice. I think glancing blows work differently on heroic elite vs epic norm, and this will complicate things: classifying glancing blows as misses should be fine as a first approximation.