Well, I will take a stab at it, but I am not an expert.
Without getting all technical, which I cannot do because down varies from year to year, bird to bird, etc., EU ratings are based on a maximum fill power of 800, with a maximum insulating value of 900. These are two different measurments.
American fill values are based on a maximum of 900.
I am not sure if the tests are exactly the same, soo, there is a minor discrepency.
Fill is far easier to test than insulating value.
Eider down is considerd the best. This duck plucks only the fully mature down feathers for it's nest. After the eggs are hatched, it abandons the nest. These are collected for down.
It is rated at 900 in both systems. Even though it will only loft to 800 in EU measurments.
Water resistance, the ability to retain shape (stiffness), weight, shape & size of a plume, maturity of the bird while plucked, care in plucking, sorting, and a bunch more variables go into selecting down. "Selected Down" means nothing. It could have been "selected" for pin feathers...lousy insulation. It could have been selected for large, mature down plumes...good insulation.
"Prime Down" means little. The majority of plumes will be mature, but it could have been taken from chickens. Not enough water resistance to count as GOOD insulation in a bag.
"Goose Down" says it came from goose. Not another type of bird. And so on.
I would suggest you buy only from known suppliers, even if you pay a bit more. Sorting down is a messy job and best handled by large sorters.
"I've also heard that the really high down values lose their volume quite fast (850+ cuin is quickly reduced to 750 cuin). Is this true?"
Well, depending on conditions, yes. A down plume is a fragile feather. A larger breast feather is used to protect it, in the case of birds. Or they are placed in low wear areas, under wings, for example. High quaility down will require more cleaning (NO DETERGENTS), periodic relofting in a dryer, shaking out before using, and fairly clean night cloths to keep them body oil free. Lower quality down has a stiffer, shorter plume to start with, hence will loose less loft over the same time. Generally, 600fill down is less subject to the effects of moisture and body oils. This is true right up to saturation with water. In which case, the 800 fill down will flatten more than 600 fill down, assuming both had the same loft to start with, and be warmer. Also, low quality down often has chopped feathers in it. The stems will not generally flatten at all, but, they also weigh a lot. The difference between 750down and 800down after about a week under normal camping conditions makes them about equivalent.
"Is the durability of these high cuin downs worse than those with a lower rating?"
No. Old down (>20 years) seems to be fine. I do not know exactly why, but I can offer this explanation. A down plume is pinched where it grows. If a fiber breaks off, it loosens at the base. So, you get a simultaneous loss of loft at the plume but an increase of loft at the barb. You get a loss of insulating value at the plume, but, an increase in insulating value at the barb. The insulating value is related to the interconnected barbules on the down barbs creating an enclosed air pocket. It seems these to are roughly equivalent in old high qualty down bags, soo, the durability of high qualty down stays good, overall.
Again, this is only a guess as to what is actually happening to account for the observed. I would gladly bow to someone with more knowledge.