I'm still relatively new so be gentle. I put Spriggins and NMG on my NB and didn't notice much difference in DPS between that and my Hundings/Endurace/Gladiators set. Is it overkill on the physical penetration? It's over 9k.
Penetration is great. Most bosses and the like have ~18k resists, so penetration will help you there. It's even more valuable in PvP where the meta has turned to Heavy Armor builds with 20-30k resists.
The end all be all is that penetration is good, as long as you have decent weapon damage too. If you penetrate a lot of the enemies armor but have low damage your damage will still be low. Now if you deal high damage but have low penetration you will have a similar outcome.
If you're running solo damage may be a better option for open world PvE. If you're in a PvE trial or dungeon group chances are most bosses will already be so low on resists due to group member sets and abilities that you'll be over penetrating and at a DPS loss. Penetration sets are something that need to be discussed before any trial run, otherwise your group as a whole might have a DPS loss.
I'm sure there's some forum warrior mathematicians out there that can give you a better analytical answer. There's also another post on the forums that people have been discussing penetration values for a few days now. Search is your friend.