Ok, here's the skinny on this:
The robe has a right-click effect of a bard song, Jonathan's Whistling Warsong.
This song has a haste component (max 25%).
This song will stack with any haste item (FBSS, Noose, SotDB, CoF, etc).
This song will not stack with the Augmentation series of spells.
This song will stack with pure Haste spells (Quickness, SLTW, Aanya's, etc). However, you will not benefit from the haste on the bard song, only the haste from the enchanter haste, providing that the enchanter haste is faster.
A haste item can stack with either a song haste, or a spell haste, but not both. The greater haste spell will be the only one that is applied.