Upon putting mods (armorings/barrels/hilts, mods, enhancements and possibly augments) into customizable (and equipped) companion's armors, your character will gain the same mods stats.
Beware that this increase will disapear as soon as you change zones. So it's not possible to use in WZ, but quite possible to exploit to solo HM flashpoints (haven't tried it though).
Also, I haven't logged on since yesterday and don't know if there were a stealth fix for it with last patch.