In fact, a kilogram of .925 will usually get you less than a kilogram of .900 US coins, because even though the pound of sterling contains more silver, the remaining .075 is unpredictable, and potentially expensive to refine out. With .900 coin silver, the remaining .100 is always copper, and that makes refining a lot easier. Also, the coins retain added value if you don't melt them (recognizable, popular). The cheapest "refining" process is the one that just moves stuff from customer to customer without even melting it.
It's my understanding that .925 coin silver is as predictable and 'safe' as .900 coin silver. The other forms of .925 silver (flatware, jewelry, etc) are the unpredictable forms of this purity category. You're right on that the best type of bullion trading is keeping the 'product' in a consistent form and not messing with 'predictable' forms. (ie melting coins just because...)
You may well be right. I haven't dug into that distinction. I've had people lowball me on .925 coins, but I think that's mainly because they wanted to lowball me.
Maybe one could make a distinction regarding the national origin of the .900 or .925 coin silver and the non-precious metal content. But that's likely someone trying to pull a fast one.