Elemental essence armour boosting:
Magical/rare/legendary metal armour: +25% resistancy
Magical/rare/legendary leather armour: +25% resistancy
Magical/rare/legendary metal helmet: +25% resistancy
Magical/rare/legendary leather cap: +15% resistancy
Magical/rare/legendary robes: +20% resistancy
Magical/rare/legendary belt: +25% resistancy
Magical/rare/legendary sarong: +15/25% resistancy (varies)
The boosts from the essences (and all boosts) are based of a combination of level and skill, this is why I have not put in raw numbers for them
But yes its great that you can stuck further resistances on items that already have them for a massive boost, good way to max them out!