Just tested shield barrier with the trinket. It does not increase bonus armor or attack power or damage. It just straight increases armor.
I have a preliminary trinket sim with relatively low iterations. T17 disabled, T18 2+4pc enabled, armor and weapons set to ilvl 725, balanced haste to about 750, crit vs mastery balanced too (crit is good up to a point, need to crit to get T18 to work good). No second trinket equipped. I also hotfixed in the gladiator mastery change from 30% to 50%.
Class trinket still isn't that good, even after +75% buff. You will be using Anzu's cursed plume and Rumbling Pebble now (RP got buffed in the last build. Cleave and burn got nerfed.)
Click the picture for a full list of trinkets. This is only the top of the list.