Originally Posted by Symphonia

Technically it's instant cast, but you have to remember instant casts use up the global cooldown, so it's better to judge it as a 1.5 second cast (assuming no haste). So you have a 2.5 second cast that does 1600~ damage or a 1.5 second cast that does 400~ damage. As for your spec, depends on what you do and what your group composition is like, really. You could go 10/0/51 for clearcasting to help with some mana problems.