+spell damage and instant spells

[TheWesson]

Hey there Lurkers.

I'm de-lurking to ask about a semi-oft covered topic: spell damage.
I know that full spell damage is related to the 3 second spell and there are deminishing returns for faster spells. However, I'm not familiar with how far the bar drops for instant spells.  Furthermore, the game clearly differentiates between true instant spells and spells that can be made instant via talents.  I say this because when I have an active curse of tounges debuff, my Arcane Explosion (formerly instant) takes cast time, where as my fireblast does not.

This whole thread is designed on end-game spec and I specifically looking at the viablility of my 13Fire/38Arcane build.  I really like it for early lvl 60 instances, it seems great in core, and works fine for me in PvP. 

I'm just not sure how to ramp it up.  Do I add +damage gear or +regen?  Right now, I'm in more of a stats build gear wise (full Tier 0 - Magisters).  I never run out of mana even constantly missling Magmadar. I'd like to find a good build of +damage and stats, but I'm not sure if the benefits of +damage can be properly realized in the arcane build I have.

I welcome all comments/suggestions.
Thanks,
-MB
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Instants are treated as 1.5 second spells (which is fair, since the global cooldown is 1.5 sec.) +Dam gets full benefit at 3.5 sec cast time, so instants get 1.5/3.5 of the damage, or 3/7, or 43%. Spells do not get extra benefit from having a very long cast time (> 3.5 sec)

Area effects are assumed to be hitting three targets, so get 1/3 of the damage boost a single-target spell would get. So Instant Arcane Explosion would get 43%/3 = 14.3% of the damage boost, all told.

I've been told that spells which can be made faster by talents (e.g. Nature's Wrath for Druids) still get the damage boost based on the original casting-time. Thus, even a talented Nature's Wrath (1.5 sec) would get the boost based on 2.0/3.5 = 57% of + dam. Spells made instant by a cooldown (PoM) still get the damage they would otherwise.

DoTs get 100% +damage spread out over their duration, assuming their duration is greater than 3.5 sec. I would guess the same to be usually true of channelled spells.

I've been told that some spells with an extra effect (like slowing for Mind Flay) get a diminished effect from +Dam. Can't recall if that's true for Frost Bolt.

Moonfire gets 43% for the instant part and the rest of the +Dam is spread out over the DoT. Something similar must be true for your fire spells that burn after impact.

edit:
People who know better than me say that spell ranks which are learned below lvl 20 get a sharply reduced benefit from +Dam/+Heal. Spell ranks learned above lvl 20 are said to always gain the full effect.