Anything You Can Do, I Can Do Better!: RDM Guide

augment vs nonaugment enhancing duration are separate multipliers so you get more duration with duration from augment instead, also you can stack skill with ghostfyre

it's not necessarily that you get more duration from augment vs. native, it's that whichever one has a lower base will give you more total duration due to how multipliers work. RDM has a very high native amount of duration + equipment, so augments of the same value end up being better.

The really interesting ones are the ones where it's not the same value. For instance, that same ghostfyre cape with only a 15% augment is still break even with an ambuscade cape for the most part for self casted buffs.