Dan,
I've listened to the vocal drum kits on the SD1 and they sound very good. What I like about the Yamaha 9000 pro and I think this is what some of the readers are talking about is the ability to play melody type playing and having different vocal layers be abled to be played on the keyboard. Being able to use velocity cross switching to change the vocal type sounds. Roland uses this really well on the new VA products. It is the same vocal sounds they used on the FP9 piano. They call these scat voices. Yamaha has added these in their new voices in the 9000pro.
On the SD1, the vocal kits you are talking about are mapped as drum voices and so are available only on drum sets and so each note plays a different vocal sound. Perhaps these sounds can be used as actual voices but I have not explored the posibility yet.
George Kaye