The 45/50 number includes the proposed changes, where the trash mobs will get -1d20 to hit. As things stand now, 45/50 is of no use whatsoever.
Currently, at 55+ you start to lose HP fractionally slower than other people, probably due to the occasional glancing blows being made on you rather than full attacks. That is the only thing that I mean when I suggest 55+ currently making a slight difference, and nothing more.
I repeat, I have never stated that AC 55+ can be any sort of decent AC in Epic, just that that's where AC, in some Epic quests and against some Epic trash (but not all) starts to slow HP loss for you. How many times do I have to repeat this, before you get the point that I'm not suggesting that AC 55 is a meaningful level of AC in Epic ? Because it isn't ?
In cases where I've been up to about AC 75 I have seen mobs actually miss me completely with greater frequency, and yes, you are absolutely right, I'm talking about the weaker mobs (although AC 75 is actually somewhat useful against the extremely weak ones, like the pet hyenas in WK). That is why I am talking about AC 75+ (currently) being needed for any sort of significant level of it. I cannot predict how much this will be affected, where the ordinary epic trash is concerned, by the new -1d20 penalties for the Epic trash ; though logic suggests that you will start to see misses at about 55+ in the new system, even though they will still be few and far between. Significant AC is also likely to be starting at 65+ instead of 75+ -- Note please, *starting* -- this does not mean that AC 75 is high enough to constitute *good* AC in Epic, because it isn't ; nor will AC 65 be in the new system.
And FWIW, yes there are many people who consider that AC which is about 10 points below minimum overall tank level AC for a quest is worthless, and they report it as such ; even where in actual fact, the lower AC may be of some benefit against some of the weaker mobs.