I did decide to purchase the px 560 despite knowing the problem with using expression; I have discovered the same issue with using an expression pedal (to a lesser degree the same problem persists with when using the mod wheel to control volume). I have found a work around that although not perfect works better for me than abrupt, uneven jumps of volume: I set the minimum value of the pedal to 1 instead of 0 and I didn't notice the annoying jumps in output volume. The down side is that the layer is never off completely; it does sound softly and can be heard. The work around for that can be accomplished by muting the zone on the touch screen.
Hopefully Casio will find a software solution to the problem, but I think I can live with this for now since the keyboard action, sounds, portability, and ease of navigation and editing are very good.
Hope this helps.