Generally there is no rule, how and when you use the performance-legato grace instruments instead of the normal performance legatos.
Looking at the "history" of these instruments, they should work better for faster movements. (and they work very well for fast passages).
On the other hand, why not use them also for slower movements. Simply trust your ears or your imagination.
I would say, there can't be enough variations and options to choose from.
Simple example: I worked on a piece where short identical legato movements were repeated very often. Here it was a really helpful to alternate between two different legato versions to get a random effect.
Regarding visible dimensions:
I think that's the problem.
Absurd that the engine could handle more than 32 dimensions, but you can't see or edit them, isn't it?
But I'm not the EXS guru here.
You know, I'm the old fashioned giga guy.
I only have this infos from our EXS team, and I don't know, if I'm really up to date.
But if I understand you right, this limitation is still existing.
Hm, a solution to see and handle more dimensions (just for mapping issues) would be much appreciated...
best wishes
Herb