Historically, PoP mobs were designed to put an end to AC soft caps that were encountered for most Velious mobs. I'd be surprised if the cap for BST's for PoP was that low.
This isn't the way that I see it. The softcap for us hasn't really changed much from when it was first put out (to replace the AC hardcap). Increases in AC are actually quite significant up to the soft cap. A 250 increase in displayed AC would show up as a large decrease in damage taken (along the lines of 25% to 30%). But, DPS dealt by mobs takes a lot of things into consideration. A mob's DPS ranges from a minimum damage to a maximum damage (these never change - aside from things like warrior discs and such). Actually damage dealt is present as a bell curve between these two end points - but the median value of the bell curve moves up or down depending on how your real AC interacts with the ATK of a mob. The median can never go above the max damage value, and can never go below the minimum damage value. Or, in other words, most of the time, if your AC is very high or very low relative to the mobs ATK you will take max or min damage.
However, for some mobs, the ATK relative to your AC maxes at some point. In Velious, for example, most mobs (except for bosses) are such that for a plate class, maximum effective AC is about 1100 or so. Any class whose softcap is at least 1100 - and who has that much AC (and this I think includes every class but the silk classes - as long as they have finished all the defensive AAs) will be just as effective as any plate class in tanking Velious most velious content. There are some few exceptions (most notably the Avatar of War). Apart from AC issues and defensive Disc, the other aspect of course is raw HPs. So for trash, my Cleric can tank anywher in Velious with no problems (and can heal himself to boot).
The original hardcap in Velious was changed to a softcap, and most classes get some return over that softcap. Now, defensive AAs (particularly the mitigation AAs) work effectively by modifying the softcap. They increase the softcap. For melee types, this is a bigger benefit earlier on, since they usually have better AC on their equipment. The casters won't see as huge a gain from this as they will from avoidance AAs - but once the casters get a sufficiently high AC, the mitigations help an amazing amount (remember that even silk classes will see a sizeable enough boost from mitigation AAs, that they will see a noticeable reduction in Damage taken).
With the softcap, several classes saw a huge gain in AC over the cap but most didn't. Eventually (as part of class balancing efforts and such) most classes got a bump in AC return after soft cap on the PC side. Our monks have something of a nerf caused by other issues. Monks originally had decent returns, and a high cap, which made them excellent tanks (of a sort) in high end Velious content. Back when I was doing Velious for the first time, our guild used monks as ramp tanks. They hadn't really gotten around to tweaking the ramp rate alot (its a proc) for most mobs, and monks had a high AC and a high evasion meaning that they were actually easier to heal as a ramp tank than warriors were. So monks were badly nerfed to change this (and eventually changed again but we missed the good modifications).
Most classes have a return above cap that makes it less than useless. For a Cleric, once we get past the eventual softcap of about 1450, we might see a real increase of AC of about 10 or 11 going from a displayed AC of 1450 to 2000. This exists on our server in part because at these softcaps (which are enhanced through AAs), it was only in elemental gear that characters saw the gains in AC which allowed them to get very high values (and thus created a need for correction).
At any rate, while PoP was supposed to correct some of these issues, what it actually does is raise the ATK on some mobs - and this goes up as you move through the content. Personal anecdotal experience suggests that the fiends in PoV level out pretty early. Cromis tanks there without much difficulty. Cromis solos in Tactics for example, but is limited to the normal ground floor ogre type mobs (the Diakus). Hendin's DPS spikes right up, and clearly Cromis's AC is nowhere near the where it needs to be for the ATK on such a mob - but our plate tanks with 1800 AC or so have no problems, suggesting (anecdotally) that there might be an effective cap for them in the 1800+ range.
BST softcap is the same now as it was in Velious (and as a leather class will sit at around 150 worn AC - this is not very high).
BST softcap will go up significantly when mitigation AAs are purchased, but it will put a softcap at around 1100 AC (displayed AC - along with all of the other leather classes). Since this is likely below the effective cap caused by ATK on the fiends in PoV, the fiend parsing is going to be fairly accurate (while it might not be for a 2K AC geared SK). On the other hand, BST's make excellent split tanks for most Luclin and Velious era mobs.
At 1100 AC, with decent HPs, a BST can tank just fine at the fiends, but they do not make good split tanks further on up the PoP ladder, where well geared Rangers will tend to do better (higher soft cap) as emergency split tanks and plate classes even better.
Finally, getting more AC isn't bad, it's getting more AC at the expense of something else. The AC will be far more valuable for a BST up to the cap than just about anything else - but post cap, for a BST who wants to tank, HPs and the related attributes will take a more prominent role. You won't want to drop below your cap to get them, but you will look more at upgrades that give better resists, HPs, and attributes than at the AC on the item. Velious gear had a lot of high AC, while Luclin had less AC and more stats and resists. PoP tends to do all of it.