Well, it has been a week, but I think I have done as conclusive testing as I can get.
I used the windower addon battlemod to isolate exactly when the spell is cast in memory. I used sch and blm casting blizzard 5 for 2 reasons. One, both jobs can cast the spell, and two, blizzard 5's cast time is exactly 10 seconds.
First observation I had was that the cast time is actually the full 10 seconds. my initial theory of casttime x .75 is way off, and it was throwing all my calculations off. Ok, here it is.
There are 3 multiplicative properties that deal with fast cast right now.
first, fast cast. This includes fast cast from the trait and gear, and elemental celerity (job trait for blm). These are all additive with each other. My naked blm was casting Blizzard V at exactly 7 seconds. Elemental celerity is 30%. Adding Nares Cap and Anhur Robe made my black mage cast at exactly 5 seconds. Chelona boots, Swith Cape, Repartie Gloves, and Loquacious earring made the cast time exactly 4 seconds.
second, Magian Staves are a separate property from fast cast. It took me a while to pin this one down. At first I thought it was cumulative with my fast cast gear because I only added it later in the testing, but I got to the point where my numbers were off by just very slight amounts and I couldn't account for it. With full fast cast gear, my blizzard V's were going off not at 2.6 seconds, but at ~3.4 seconds there abouts.
third dark arts and alacrity. One of two things happens here. 1. The bonus for alacrity is 50% and it overwrites dark arts 10% bonus. 2. alacrity is only a 40% bonus and it is cumulative with dark arts. Either way, testing with 20% fast cast on blm. dark arts fired off at exactly 4.5 sec. alacrity at 2.5. With 30% fast cast, dark arts was casting at 3.6, and alacrity at exactly 2.0.
Lastly, with all my fc gear and my staff and alacrity, I was casting at 1.7 seconds. Because that was exactly what I was expecting, I can say with 100% certainty that alacrity breaks the 80% cast reduction cap. I am pretty sure this is already common knowledge though, as sch stuns strats in Legion already abuse this, only on the recast side. Just confirming it. So I have reworked my equation.
Time to cast = Base Cast Time X (1 - fast cast gear and traits) X (1 - magian staff cast bonus) X (1 (Dark Arts + Alacrity))