For those that don't follow this stuff, Hurk (a cleric) actually posted some useful information and charts on the eqlive boards at
http://eqforums.station.sony.com/eq/boa ... .id=128241
The basics are that he parsed against himself on test with the same (within 2) AC against a certain mob.
With all gear-based (no spell or shield) AC, 16.42% of hits were for min and 14.73% for max.
With gear- and spell-based (no shield) AC, 16.49% of hits were for min and 14% for max.
With the same AC coming from gear, spells AND shield, 19.49% of hits were for min and 11.25% for max.
So, even with the same AC, having some of it from shield is a plus, as shield AC effictively raises the softcap, in that it is not mitigated like the rest (gear, spells) are. This confirms what has been explained here. Unfortunately, it seems to demonstrate that buff AC is NOT overcap.