Specialized Expedition followup:
I’ve just done a 13 mile (20k) trip on this saddle, some of it on fairly rough forestry tracks, and I can still feel my backside 
. An unexpected benefit was that the springs turn out to be very effective at deadening most of the vibration from the hardpack of the tracks, and made them eminently cyclable. Not sure how long the tyres will last if I do this too often, though.
Definitely a good investment, and I can heartily recommend it.
Colin