Congrats on finally figuring out RoP actually does damage, instead of being ignorant and denying it doesn't do anything like SB did for MONTHS.
Almost as bad as you claiming RoP bonus was impacted by Health% for MONTHS.
Proto you were very much correct man. I definitely prefer the Armor Pen now. Have it at +54% it is fantastic
Why did you use +54% Armor Penetration? I never recommended you do that. But, with 320 willpower...
+27% Armor Penetration and +27% Critical Damage > +54% Armor Penetration against enemies with less than 100 Armor
+54% Armor Penetration > +27% Armor Penetration and +27% Critical Damage against enemies with more than 100 Armor.
Update on AP. It was good when I had high health. Attack% worked out better for low Health. Will be switching back to attack%
Edit: so yes. After further testing with it I found:
Armor Pen is good for high health play throughs, so not the build in this thread that I use.
Attack% is good for my build when playing at low health.
Conclusion: use attack% attachments when playing Reaver the way I do. Armor Pen if you like staying at high health when playing in NM
Bruh, your conclusion is balogne. Oh great scrub-lord. 
Let me guess...you switched from +54% Armor Penetration to a +16% Attack and +27% Critical Damage upgrade?
I've already explained that the damage formula is technically two separate parts. As health drops, the damage_multiplier gets larger, meaning each increase in attack becomes less of a percentage increase. +Attack and lower health bonus impact the same modifier.
The portion of the formula that is impacted by Armor Penetration does not change based on Health%.
With 320 Willpower:
+16% Attack and +27% Critical Damage > +27% Armor Penetration and +27% Critical Damage against enemies with 0 Armor.
This is obvious because armor penetration is useless against enemies with 0 armor. But, that 16% attack is only +293 damage per Dragon Rage. That number doesn't change at all, regardless of your health or what swing...it's a flat increase. This means you actually get the most benefit out of it when you are at full health. Completely the opposite of your conclusions.
+27% Armor Penetration and +27% Critical Damage > +16% Attack and +27% Critical Damage against enemies with more than 50 Armor
Armor Penetration's advantage over attack isn't really noticeable at 50 Armor. But once enemies have 100 armor you'll see about +270 damage per Dragon Rage at full health and up to +674 damage per Dragon Rage at 25% Health on the 3rd X2 swing. Hmm, that seems to be the complete opposite of your conclusions as well.
Armor Penetration's advantage over Attack only gets bigger as enemy armor increases.
I know what I will be suggesting for the future.
What you should suggest depends on the promotions and gear of the user you are making the suggestion.
If the user has low cunning, low willpower and no Hakkon weapon, critical chance or attack is best.
If the user has low cunning, low willpower but has a Hakkon weapon, critical chance is best.
If the user has high cunning and a Hakkon weapon, critical damage is best.
Whether Attack or Armor Penetration is better as the secondary upgrade depends on the users willpower.
Less than 200, probably best to use Attack.
More than 200, probably best to use Armor Penetration