No can do, only melee and bows have that kind of damage potential and actually remains silent. And magic has no sneak attack modifiers.You have to abuse the restoration glitch with alchemy to do that kind of damage with spells.
The illusion perk only makes the casting of spells silent; the hit effect is not.

I beg to differ--I've already used a magesassin before who specialized in Alchemy, and I had little need for the potions. I also managed to stay stealthy during "sneak" attacks. Furthermore, the mage had 100 Pickpocket (really easy to level that thing up, honestly) and Poisoned, so I was both able to, without a weapon, make enemies weak to an element or magic (or both) and fortify Destruction, but I got by fine without doing either. I eventually deleted him because I don't like Dunmer, though.
This time around, though, I want to be able to do the same thing but without focusing on actual stealth skills.
Edit: Because I had access to Muffle and Invisibility (and later muffled equipment, thanks to the DB), I did not invest in Sneak at all (though the skill raised to about 60 or 70 pretty easily). Despite that, I was able to fire off multiple spells without being detected, including Destruction spells.
I know there's no sneak bonus for spells, but I find that pretty balanced given their possible effects. Even if you're just going to deal raw damage with a stealthy magic attack, you can hit multiple enemies at once (something traditional stealth characters cannot do) and immediately cast invisibility when you're done. I find a stealth mage superior to a melee one; a melee stealth character (even one with a bow) generally has to take breaks between targets to cool down the searching period and stay hidden. I never had that problem; my character was not one with the shadow, he
was the shadow
