Experiments on Fireball Cast Speed.

All tests are timed using the windows clock. Lag effects are assumed to be negligible. Reaction and button pressing time assumed to be negligible. All times have an error of 1 second.
Control (everything is compared to this)
Test: 1032 INT, CC+4 Fire wand, no Vivace.

• Result: Fireball cast time is 18 seconds.

Effects of INT on cast time
Test 1: 0 INT(Via Potion Poisoning), CC+4 Fire wand, no Vivace.

• Result: Fireball cast time is 23 seconds.

Test 2: 1492 INT(INT roll, r1 Mind of Chaos + Claudius' Conspiracy), CC+4 Fire wand, no Vivace. (This is to make sure Potion Poisoning wasn't the thing slowing down my magic)

• Result: Fireball cast time is 17 seconds. (Damn. Well, that doesn't change the fact that my normal cast time is 18 seconds.)

Conclusion: Either Potion Poisoning increases cast time or Int really does reduce cast time. Someone with 500 or less int should test this. Also, if INT does boost cast speed, someone should try to graph it.
Ascertaining the right formula for wand cast speed boosts
Test: 1032 INT, +36% Phoenix fire wand, no Vivace.

• Result: Fireball cast time is 13 seconds.

Conclusion: This is closer to the result expected if the formula was (Base cast time)/(1+(Wand Speed boost)) rather than (Base cast time)*(1-(Wand speed boost))
Ascertaining the right formula for Vivace magic speed boost
Test 1: 1032 INT, CC+4 Fire wand, r1 Vivace + r1 Instrument Playing + Allegro non-upgraded Mandolin (12.98% vivace boost total)

• Result: Fireball cast time is 17 seconds. (Gah, I can't get any data from this.)

Test 2: 1032 INT, CC+4 Fire wand, r1 Excellent Vivace + r1 Instrument Playing + Allegro non-upgraded Mandolin (14.28% vivace boost total)

• Result: Fireball cast time is 16 seconds. (Dammit! Still inconclusive!)

Conclusion: Both (Base cast time)/(1+(Vivace boost)) and (Base cast time)*(1-(Vivace boost)) could account for these results. For the next test, I'll assume its the former.
Determining how wand upgrades and Vivace stack
Test 1: 1032 INT, +36% Cast speed Phoenix fire wand, r1 Vivace + r1 Instrument Playing + Allegro non-upgraded Mandolin (12.98% vivace boost total)

• Result: Fireball cast time is 13 seconds. (I noticed a small boost, practically negligible.)

Test 2: 1032 INT, +36% Cast speed Phoenix fire wand, r1 Vivace + r1 Instrument Playing + Allegro non-upgraded Mandolin (14.28% vivace boost total)

• Result: Fireball cast time is 12 seconds.

Conclusion: These results are inconclusive. It could support either (Base cast time)/(1+(Vivace boost)+(Wand Speed Boost)) or (Base cast time)/((1+(Vivace boost) * (1+Wand Speed Boost)). Someone with a better timer and more extreme vivace effects, please test this out.

Effects of potion poisoning
Test: 632 Int(Thanks, Belldandy517), CC+4 Fire wand, no Vivace.

• Result: Fireball cast time is 20 seconds.

Conclusion:Potion poisoning does not seem to directly affect cast time.

Effect of Magic Weapon Mastery
Test 1: 1500 int, CC+4 +20 magic attack Savage Fire Wand, no vivace

• Result: Fireball cast time is 15 seconds.

Test 2: 1500 int, +36% Phoenix Fire Wand, no vivace

• Result: Fireball cast time is 13 seconds.

Conclusion:What the...?
As of 14 Mar 2014, these numbers have not changed.

I'd like to note that the cast speed formula appears to have changed when Druid was implemented on our servers. I also heard rumors that it was going to change again when Diva is implemented so that reforges, cast speed upgrades, and vivace can all stack together properly. From my testing, vivace does not appear to affect cast times at all if you have a very high cast speed reforge.

Thanks for the heads up. I'll wait a bit before doing my testing then.
That being said, what do you mean by "All stack together properly"? Are some variables being ignored in equations or something?

Well my last sentence was supposed to clarify what I meant but upon rereading I guess that was pretty vague.

As far as I can tell, the game currently checks if the bonus from vivace or the bonus from reforges gives a faster cast speed and chooses accordingly. This means if you have a very high level speed cast reforge, the game will ignore any bonus from vivace. I tested this with a crown ice wand with speed cast level 20 though and the casting speed of icebolt and ice spear is very short in general (especially because I now have over 1200 int naturally) so it may have just been hard to detect a speed increase.

One of my mage friends mentioned that an update in Diva should make vivace + speed cast reforge more noticeable.