Ok so this thread is designed to demonstrate how spell haste stacks for players and more importantly in PvP. Since most of the competitive PvPer's in arenas today that multi-box are running shamans I will use shamans as my theory crafting design. I also advise you scroll down to the TL;DR section unless you want to learn how Spell Haste is calculated...

First the wiki on spell haste for formulas. http://www.wowwiki.com/Spell_haste

Second a spell haste calculator online to help test my terribad maths http://wowhaste.co.nr/spell.php

So we have the base casting time of a level 70 Elemental Shaman's Lightning Bolt at 2 seconds and a Chain Lightning is 1.5 seconds.

Bloodlust increases spell casting time by 30%. 1% spell haste = 15.75 Haste Raiting. Bloodlust also does not affect the GCD (Global Cooldown) as the stat Spell Haste does.

With Bloodlust only activated we can achieve a casting time of 1.54 seconds on Lightning Bolts and 1.15 second Chain Lightnings.

Per the wiki keep in mind how spell haste is stacked:

Note: Haste rating stacks additively with itself but haste stacks multiplicatively. That means that if you have 157 haste rating, you will have +10% haste, no matter how many sources and items that haste rating comes from. If you then use troll Berserking for +30% haste, you would have 110% * 130% = 143% haste. - Source www.wowwiki.com/spell_haste

Now we have Bloodlust going and we stacked haste on our gearset putting us at a realistic 140-150 spell haste. (You can get more via t6 content gear and drops, but unless you have 4 shamans doing that content every week, we'll just consider the "Typical" player in full Venge/Vindicators/Guardians rewards.)

To make math easier, let's just go with the above example of 157 haste raiting. Now we take into consideration 10% spell haste from haste raiting + 30% from Bloodlust which will take us down to 1.4 second Lightning Bolt casts, and 1.05 Chain Lightning casts. If we go on to stack another 10% haste if you are a troll shaman that would put your casting speed at 1.26 for Lightning Bolts and .95 for Chain Lightnings.


TL;DR Section (TL;DR = Too Long; Didn't Read)

Base casting time of Lightning Bolt = 2 seconds

Base casting time of Chain Lightning = 1.5 seconds

Elemental Shaman with Bloodlust only:

1.54 Lightning Bolts and 1.15 Chain Lightning

Elemental Shaman with Bloodlust + 10% Haste stacked from items (157 Haste Raiting):

1.4 Lightning Bolts and 1.05 Chain Lightning

Elemental Shaman with Bloodlust + 10% Haste stacked from items (157 Haste Raiting) and troll racial "Berserking 10% casting speed at full HP:

1.26 Lightning Bolts and .95 Chain Lightning

In my opinion to stack haste over spell damage for PvP is a bad idea after seeing the numbers on paper. If you are troll I still think it has very limited use except for the novelty of being at 40% hp sometime and blowing all CD's to achieve maximum spell haste for a good recording moment.

Also I'm not sure that the amount of spell haste gained would acutally drop the GCD down to under 1.4 with your spells as well. So basically even if you picked a troll and had 1.26 LB's, the GCD would still be greater than 1.26 so you would be capped at the GCD casting speed from chain casting them.