PvP only (you do 15000 tooltip direct DMG and 15280 debuff)
- at 15000 armor, you will have ignored their armor. If they have 20% in ironclad and 10% in Hardy, they will have 64% total mitigation. (5544 DMG)
- at 18000 armor debuff, same buffs, 65% total mitigation (5316 DMG)
- at 21000 armor, same CP buffs, they will have 67% total mitigation. (5065 DMG)
- at 26000 armor, same buffs, they will have 69.8% total mitigation. (4646 DMG)
Now let's look at a 10k debuff:
- 15000 armor, 66% mitigation (5125 DMG)
- 18k, 68% mitigation (4874 DMG)
- 21k, 69% mitigation (4623 DMG)
- 26k, 72% mitigation (4204 dmg )
And now a 8k debuff:
- 15k, 67% mitigation (4958)
- 18k, 69% mitigation (4706)
- 21k, 71% mitigation (4455)
- 26k, 73% mitigation (4036)
Based on that, it looks like a 2% reduction to their total mitigation. But looking at the DMG hurting your 15000 resist target, adding 5280 pen will add about 419 DMG to your burst number. Major breach/fracture will give you that extra DMG.
While a low amount, if they didn't use a shield or high impen, you could be looking at a higher initial DMG and in turn a higher burst. If you try to stack higher than 10k, without a debuff mechanic, you'll have to choose between reducing elfborn or reducing your dmg mitigation stars. At high CP levels you might be able to give up some points, but overwise you still need 155 points total for minimum offense (and 75 points left over)
Conclusion:
10k pen in PvP seems to be ideal. After that, you'll give up too much DMG/crit to capitalize on the DMG bonus and before 10k you'll do less DMG on the target. This means less crit and less burst DMG. Using a debuff spell will give you extra burst without giving up on stats so that's why I say aim for 10k pen in PvP.
I'm sure someone can write a better summary (especially with showing exactly how increasing the DMG is at 7-8k pen compared to using 10k pen and a 5280 debuff), but that's how I feel about penetration versus DMG in PvP. As a Templar my DMG tooltips are not high compared to nightblades, so taking spell erosion away and adding another 2-3% in tham/master didn't make much sense since adding 5280 pen was giving me more DMG.
Also shields/block weren't calculated. But then again, shields can be cc'd to prevent recasting and with the changes to some abilities going through block I imagine block not being much of an issue.