I don't know about frequencies that are "supposed to" be there (no rules, after all!

) but you will lose some of the highs with a 250k. And I don't know if it makes the pickup quieter, but it may "seem" quieter.
On the other hand, I've seen it claimed that a 250k gives you a smoother roll-off when you turn down the volume. There are less highs on 10 than a 500k, but you don't
lose as many highs when you turn down.
I'd use a 250k on an HSS guitar, because I don't like the sound of single-coils with a 500k.