I've been having this discussion with several people in and out of guild and none of us can quite seem to agree on this issue.
I have 55 passive spell haste, and if my Scarab of the Infinite Cycle procs (giving me an extra 320), it lowers my Flash of Light cast time to 1.27 seconds. Logic would dictate that I would be clipping the GCD on every cast if I'm casting with less than 200ms ping. I've been testing this out in Shattrath for the last half hour, and I haven't clipped the GCD a single time when my scarab procs.
Has anyone else done any research or had any concrete facts regarding whether or not spell haste actually lowers the global cooldown or not? From what I am seeing here, logic would dictate that it does because I should be getting "GCD owned", so to speak, when my scarab procs.
Turn your combat log on, spam yourself with flash of lights during scarab procs. Check combat log to see if they occurred within 1.3 seconds or so of eachother, or if they were 1.5+. Stopcasting macros probably would be a must.
Sadly, I've managed to avoid getting the scarab myself - but tests with around 100 spell haste were inconclusive (only had FoL #2 hit < 1.5 after FoL #1 once per 100 casts or so - which could be an aberration).
All of your spells are initiated manually: You decide when they start, and they automatically go to completion x seconds later. If you are a button masher, you will mash your flash of light button at most maybe 4 times a second, meaning that no matter whether you have 0% haste (1.5s FoL) or 15% haste (1.275s FoL), you still initiate spells at discrete intervals, and since both cases could fall under the same 0.25s interval, you might not see any benefit from haste.
If you aren't a button masher, well chances are you just aren't pressing your FoL fast enough to truly benefit from your haste. That is why spell haste is so flawed compared to melee haste.
It has been determined (see Working Theories of Theorycrafting in the Class Mechanics forum) that spell haste does not lower the GCD, but that bloodlust and heroism do lower it. I do not know, however, whether the meta gem counts as spell haste (and thus has no effect on the GCD) or whether it falls into the bloodlust/heroism category however. (I would guess the former but I don't have a character with the meta with which I might go about checking.)
I do not know, however, whether the meta gem counts as spell haste (and thus has no effect on the GCD) or whether it falls into the bloodlust/heroism category however.
As shown in the post a few above yours, the meta gem is spell haste. The bloodlust and heroism seem to be the only player abilities in that category (Thottbot World of Warcraft: Spell Effect: Apply Aura: Haste (Melee, Ranged and Spell) %). Interestingly, there do seem to be several mob abilities in that category though, some of which are debuffs. Might be worth checking to see if these debuffs actually increase the GCD, which would be rather harsh.