Posting my 3 initial tests to find the softcap. They are all against
Shiel Glimmerspindle, 1000 hits each:
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100 AC Test, No Item in Secondary Slot
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DV, Count
19, 121
22, 33
25, 33
28, 39
32, 36
35, 46
38, 31
42, 35
45, 38
48, 38
52, 44
55, 41
58, 35
62, 38
65, 36
68, 30
72, 36
75, 42
78, 31
82, 217
Total Damage = 53149
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150 AC Test, No Item in Secondary Slot
=========================
DV, Count
19, 216
22, 35
25, 39
28, 38
32, 30
35, 40
38, 36
42, 34
45, 30
48, 44
52, 32
55, 49
58, 40
62, 37
65, 43
68, 39
72, 37
75, 43
78, 32
82, 106
Total Damage = 47028
=========================
200 AC Test, No Item in Secondary Slot
=========================
DV, Count
19, 289
22, 41
25, 41
28, 33
32, 32
35, 43
38, 43
42, 38
45, 40
48, 33
52, 38
55, 38
58, 37
62, 39
65, 33
68, 39
72, 41
75, 41
78, 34
82, 27
Total Damage = 41805
So far this suggests the softcap is not between 100 AC and 200 AC. The damage difference between 100 AC and 150 AC is similar enough to 150 AC and 200 AC.
My 363 AC tests Show that the damage I took at 363 AC without a shield was 39236.
Going from 100 AC to 200 AC reduced my damage by 11346.
Going from 200 AC to 363 AC reduced my damage by 2569.
There is almost certainly a softcap somewhere between 200 AC and 363 AC. I'll try to pinpoint it when I run the next few tests.