Bonus conferred by fast casting staff modifier

So how much does the "Improves spell cast time" (change: X%) improve your cast time?

Does it make it instant-cast? I've been looking at descriptions of the modifiers posted in various places, but haven't been able to find anything that actually states what the bonus is.

The reason it matters is that my primary is a mesmer. If the staff bonus is something like "reduces cast time of any spell by 1.0 seconds" or "reduces cast time of any spell by Y%" that's not too bad. But if the bonus is "makes spell instant cast" then it's somewhat less useful, as it's partially redundant with respect to my innate fast casting attribute.

Well call me crazy, but wouldn't casting spells instantly be better than some kind lesser reduction like the ones you already mentioned anyways?

from what i've seen i'd say that the "improved cast time" is a 50% reduction in the needed time. Ice spikes has a 2 second cast, and when the improved cast time does kick in, there's no cast bar, just the animation of a 1 second cast.

same applied to water trident, went from the 1 second cast to 1/2 second.

as for the percent its your chance out of 100% (or 1 in 5) to improve your cast time.

It does appear to be 50% of the base cast, although the prima guide says 20% (but most people agree that the guide is riddled with errors, so...).