While it's true that additive damage effects only occur on the first attack, the only current way to give a mage a second attack is Battle Fury. Since Battle Fury is a separate action, no reduction takes place. If there WERE a card that granted Double/Triplestrike then it would.
Exactly. Battle Fury allows you to roll all dice again including all bonuses. When you make multiple strikes in the same attack (such as Doublestrike or Triplestrike) then any bonuses only apply to the first attack.
Same goes for ranged attacks. +1 to Lightning attacks (like from the lightning ring) only adds to the first shot of a Chain Lightning.