The Parasitic Impetus/Epidemic infections cause enemies to take 7% more damage from all sources(in addition to the shrill spawn on death). It does not apply an armor penetration effect.
Internally, we have been working on adjustments to the way the PvP weapon stat modifiers are applied inside battlegrounds and shadow wars. The old flat 65% damage reduction and 2.5x Critical...
We have tested Sustained Suppression internally and are unable to reproduce the issue you are describing. As stated previously in the thread, Sustained Suppression requires the weapon to be equipped...