I've parsed my log against A Planarian Larvae near the zone in of pov from ac 1000 to 1700, here is the result:

ac 1000:
avg hit 208
dps 99

ac 1100:
avg hit 194
dps 92

ac 1200:
avg hit 181
dps 85

ac 1300:
avg hit 169.27
dps 79.29

ac 1400:
avg hit 173.63
dps 84.54

ac 1500:
avg hit 167.70
dps 74.68

ac 1600:
avg hit 164.23
dps 83.18

ac 1700:
avg hit 162.17
dps 75.43

I debuffed him with Hand of Ro and Ro's Illumination. I
took off my equips to make my ac lower. I guess the weird
data in ac 1400 is just a peak value which happens
sometimes, so let's ignore it. DPS doesn't means much in
this case since ac is mitigation only, so we should look at
the change in average hit.
Apparantly the benefit from ac decreases significantly after
it passes 1350~1400 mark. However, it may caused by the
mob's lv is too low and 1350~1400 is just the ac cap
against that single mob, so I went to plane of water and
tested it with A Triloun Champion. I debuffed it with slow,
criple, and 2 Ro's. Here is the result:

ac 1150:
avg hit 361.71
dps 123.36

ac 1276:
avg hit 347.61
dps 102.91

ac 1378:
avg hit 288.5
dps 81.24

ac 1456:
avg hit 328.05
dps 99.36

ac 1556:
avg hit 328.02
dps 110.58

ac 1685:
avg hit 358.02
dps 114.26

I spent less time on this test(about 5~7 mins in each
interval), so there are more peak values and seems a little
unreasonable. However, we can still find that when the ac
passes 1350~1400 mark, the benefit from ac decreased
significantly again. The chance that 2 mobs from different
zones and with different levels but have the same ac cap is
pretty low, so with high possibility it's the ac soft cap of
druids, not those mobs'. If this is correct, then the ac soft
cap for druids is somewhere from 1350 to 1400 in POP. I've
not tested in higher level zones like GOD and OOW, and
I'm not sure if the soft cap will be higher in GOD and OOW
zones.

Well, when you debuff the mob's ATK, you are basically throwing your calculations off (not by much, but still.) The benefit of AC is figured out against the ATK of the meleer, not so much the era of the mob. If you want a more manageable parse experience, slowing the mob wont affect things as far as max hit and avg hit go.

A final number around 1350 wouldnt surprise me, since that's what monks got capped at.

It's also dependent upon your AAs (and possibly level?). The Combat Stability/Innate Defense/OoW-version-of-Innate-Defense line of abilities supposedly increases the AC soft cap.

At least one developer also claimed that armor location is related. I remember reading that AC from a shield (might have to be tagged as a shield?) has full effectiveness regardless of where your AC is compared to the soft cap.

Still, as a general guideline, seeing a point of diminishing returns around the 1300s isn't surprising.