Any stamina nb will outperform this by far. The problem is you have less burst together with less endurability. For the destro staff heavy attack from stealth you don't get a stealth bonus as far as I remember. You can't simply burst your victim down to execution range. So you have to mitigate dmg and hinder your enemy from recovery. Most decent players can reduce your healing to more or less 0. So you need to be able to do that too and additionally dodge, block, shield or mitigate dmg to survive ;-) You can't really block or dodge that much so you need to shield or mitigate.
I would put at least a shield into my bar and slot incap for more burst and heal reduction. Then you need a siphoning ability in both bars to buff your magicka. You have it only on melee bar. I'd use swallow soul and/or cripple. You need a decent soft cc anyway. Spectral bow isn't really useful as snare. Swallow soul does dmg and is cheap. You lack heals anyway. You wouldn't need a resto staff for that.
You can be an assassin as magicka nb but its not like 2h/bow where you nuke your victim down before they can sort their fingers. Its more about bringing the enemy in a vulnerable situation with your opening and then siphon him down.