Quite a few stats from the equipment carry over to the shapeshifted form, but not everything. The exact mechanics are pretty complicated.
Many defensive bonuses carry over, but offensive ones generally do not. The armour rating carries over if it is above the default value for the shapeshifted form.
But a very significant disadvantage is that you lose your sustained spells and cannot cast new spells if shapeshifted. A mage is much more useful if he can cast spells, so staying shapeshifted is not very effective. Shapeshifting when running low on mana would be useful if it didn't take so damn long. Mana potions are easily made, so taking some potions and casting more spells is the better strategy.
Shapeshifting is generally considered the worst specialization for mages, while the Arcane Warrior is considered the most powerful.