Made a small test today.
Scenario 1:
Armor: 1790
Ethereal Armor: 500
Flame Res: 1035
Spell: Fusion Burn, Perfect Spell, Multicast, Fiery Strike V
Damage: 2153
Scenario 2:
Armor: 2383
Ethereal Armor: 0
Flame Res: 40
Spell: Fusion Burn, Perfect Spell, Multicast, Fiery Strike V
Damage: 3249
Scenario 3:
Armor: 1790
Ethereal Armor: 500
Flame Res: 1035
Spell: Perfect Spell, Multicast, Fiery Strike V
Damage: 1161
Scenario 4:
Armor: 2383
Ethereal Armor: 0
Flame Res: 40
Spell: Perfect Spell, Multicast, Fiery Strike V
Damage: 1368
comparing scenario 1 and 2 would mean only difference of 50.90% (if I calculate right)
comparing scenario 3 and 4 would mean only difference of 17.83%
Somehow my mind is screwed now what I should think about it...