Last I heard, FC is limited to a recast reduction of 40% (80% casting cap, 80% FC = 40% recast). But you can also combine it with gear haste (25% reduction), the spell Haste (15% reduction), and possibly other forms of haste such as Haste II (30% instead of 15%), Geo-Haste (varies, but can be up to 35% with 900 skill tier and Dunna, or more with Idris), or Marches.