This is my take on the new Shadow changes from the perspective of a Shadowmage, I'm really digging the changes first of all. My Shadow is a higher level with more HP and it seems he can cast not only my Greater Spell Focus spells, but my slotted Cantrips and even my Specialist Wizard spells. It seems a lot like the Simulacrum spell from Baldurs Gate 2, though there are a couple issues I've experienced.
It takes the spot of a summon which is a hard sell when you're looking for something to tank and have any DPS, maybe if it counted as a henchmen instead. For the spells, they just fire them off randomly and using every spell it can, it will spam Color Spray and Ray of Enfeeblement on undead, or Burning Hands which will destroy all loot it hits, and I can't tell but I'm pretty sure the DC on these spells is pretty low. Maybe a command or dialogue option to tell your Shadow what spells they can/can't cast.