A few years back i spent a week or so on the 6th, but it was way beyond my powers as i expect the 8 others were.They really are for pianists of the highest order.
I wonder how much the difficulty of them is derived from katsaris making his recording of them. After hearing him play like that it's easy to lose sleep. As a result we might over estimate them. That's my theory, but I think there's only one way to find out. I'd like to give it a shot, they're such incredible music.