When it comes to spell haste, it seems blizzard is moving in this direction for casters based on all the new loot from ZA and badges.
With all the haste gear from ZA and badges I came up with 213 spell haste. (Dunno if I missed anything but if you feel up to it see what you can come up with). I looked around online and actually found the formula to calculate the casting speed reductions based on you spell haste rating.
New Casting Time = Base Casting Time / (1 + (Spell Haste Rating / 1570))
I'll use a fireball with the above stated 213 spell haste as an example.
X = 3/(1+(213/1570))
X = 2.64
If you're wondering where the 1570 comes from I'll explain that next. 1570 is the amount of haste it would take to reduce a 3 second cast to 1.5 seconds (GCD).
X = 3/(1+(1570/1570))
X = 1.5
That being said, 1570 is the "cap" on spell haste for 3 second spells. Of course spells like scorch or arcane blast would be well below the GCD at this point, BUT, since you can't bypass the GCD you would still have to wait until the spell is ready to cast again.
I figured with the addition of the Icy Veins talent I should do some number crunching to determine how much haste rating Icy Veins gives you while it's active. With 0 spell haste Icy Veins takes a 3 second fireball to 2.5 seconds. To figure out how much spell haste this adds up to you use the reverse formula.
Spell Haste Rating = 1570((Base Cast Time/New Cast Time)-1)
X = 1570((3/2.5)-1)
X = 314
Let's see what 213 spell haste with Icy Veins does to a fireball cast.
X = 3/(1+(527/1570))
X = 2.25
I also did some searching to figure out a rough answer to the question of which is better: Hit vs Haste vs Dmg vs Crit? From what I can see in most posts Hit > Haste ≈ Dmg > Crit. I found a really well written post by a guy on the Subcreation forums that I'll post below.
17 spell haste = 1.08%
Originally Posted by Tiga from Subcreation
Source of Formulas: http://www.wowwiki.com/Spell_haste
Source of Post: http://forums.subcreation.net/viewtopic.php?pid=73006