Was tested over at KI... so I ran my own tests, and found this to be true. There is a seperate haste cap from Magic haste + Eq Haste.
Recast for spells are capped at 50% from Haste(Eq + Magic). Fastcast will reduce this beyond the 50%.
While weakened, recast for spells are affected by Eq Haste + Fastcast, but unaffected by magic haste.
Eq Haste caps at 25%. (Catastrophe counts as Eq Haste)
Magic Haste caps at 43.7%ish. (Haste Belt counts as Magic Haste)
Total attack delay reduction from haste adds up to 68.7%ish.
Job abilities which affect attack delay (Dual Wield, Martial Arts, Hasso, Desp Blows) work ontop of the capped delay reduction from haste.
I'm unable to determine precisely the capped total attack delay reduction from haste + job abilties. I know some people have tested Ninja Dual Wield cap, and Xno has done a bunch with Scythe.