Alright so I did the test myself since it seems you didnt do it. Turns out agro seems to be fine.
First off, agro is generated per swing, not based on damage...
Notice 5 swings, different damage, same hate value.
Secondly, agro mods like
Echo of Anger, do indeed work:
Notice 5 swings again, this time hate is 100. 89 * 15% is 102, but its close enough that I would say its working.