[Sabaron]

Let's just calculate it:

Stun takes 0.5 seconds to cast.

BLM/RDM: 15% Fast Cast (from Fast Cast II)
RDM/DRK: 30% Fast Cast (from Fast Cast III + Chapeau)


BLM/RDM: 0.5 * 0.85 = 0.425 seconds
RDM/DRK: 0.5 * 0.70 = 0.35 seconds

So a RDM/DRK has a 0.075 second lead on on BLM/RDM. I wouldn't discount that considering that since Gospel of the Lost is very fast we're dealing with human visual reaction time which is (according to wikipedia) around 180-200 milliseconds or 0.180 to 0.200 seconds. Therefore the bonus RDM/DRK has of BLM/RDM is 37.5% of the human visual reaction time which is much more significant when you look at in as a percentage.
I know that it seems like a very small amount of time, but when we're hovering over a button to activate a readied spell, that's really what we're dealing with. A fraction of a second wins a race and it can be instrumental in fine actions like Stunning.

Also don't forget RDMs can also wear Duellist's Tabard that pushes Fast Cast to 40%.

0.5 * 0.6 = 0.3 seconds which is a full 0.125 seconds faster than BLM/RDM and is 62.5% of HVRT.