As the title states, the slotted Wing Staff does not reduce fixed cast time at all. I tested this using the @showcast command.
Wing Staff  (Item ID: 63552)
Without Wing Staff :
With Wing Staff :
The chat log indicates that with and without the Wing Staff, the cast time remains the same. The skill that was used in this test was Lord of Vermilion with a base Int of 200, and a base Dex of 150.