As I remember:As far as I remember for mono katanas and bows in the PnP they reduced armor to 1/4 effectiveness and any damage that got through did full damage. For armor layering it was usually restricted to 2 layers with an encumbrance value that directly subtracted form your REF score which directly impacted your ability to fight. Also there are a lot of house rules for staged penetration of armor and armor degradation.
Mono weapons reduce soft armor by 2/3 and hard armor by 1/3. Powered mono weapons reduce soft armor by 3/4 and hard armor by 1/2.
Blades reduce soft armor by 1/2. Arrows with broad heads doubled damage after armor, spinner arrow heads tripled damage after armor.
Armor stacking was changed to a variable system. Some armor has encumbrance penalties in addition to the weight. Metalgear has a penalty of 2. That means just by wearing it the character's ref is reduced by 2. That means all the character's ref skills is also reduced by 2.
Armor's effectiveness was reduced by 1 each time it was penetrated.