Rob – I think the sections in the middle descent are good, but its too short and then flattens out into a gravel track, but you may like that kind of thing 🙂
Causeway is challenging due to the 'Babies head' size rocks which are best handled at speed. The middle rocky section depends on line choice, either left or right, however straight down the middle may be a goer.
Plantation is technically more difficult but once past that hairpin (which is quite easy once you've done it) its a case of holding on and controlling speed.
As the causeway is longer and rougher, I'd say its harder overall than the Stanage plantation.
Blacka Plantation (assuming you mean from Devils Elbow) is good, but not that difficult.
I think the problem with the grades are that most of the routes in the books require a degree of skill to get down cleanly and when you compare them all with one another there is little difference to differentiate them.
There are probably only 2 or 3 proper black (or should they be double black?) descents – Cavedale springs to mind.