Mobs in dungeons and trials have 18,200 armor.
Each gold sharp weapon is 2,580 penetration (5160 with both).
Two Fanged Snake is 5,000.
Major Fracture (most common debuff and basically always up) is 5,280.
That means that these 3 (de)buffs are up 100% of the time for a total of 15,440 armor, which means you have 2,760 armor left to penetrate.
If you have literally no other penetration tool than NMG (2,580), you are basically at the penetration cap, but realistically, in a group situation, you can have Alkosh (3,010), the Crusher enchant from tanks (I think that was 1,600~ not sure about this one) and Minor Fracture (1,380) coming mostly from Power of the Light from templars.
So, to answer your question, NMG (or even Kra'gh) will be overkill in organised groups, if you're doing dungeons with randoms, you can technically use NMG. So, yup, you'll have to know what's in use in your group to avoid overpenetration.
EDIT: And basically no CPs in physical penetration obviously.