What do you think? I heard it's quite difficult. Maybe I'll bullshit my way through the rly tough parts; most people won't notice 
And I would like to play the piano like Fredrik Ullen, but it ain't gonna happen. Have you even
looked at the score? The latter part of the piece (a good bit of the total) is written on up to 5 staves without any obvious reason. I imagine it would take quite a while to do your own transcription in order to make sense of the chords, but you're gonna fake it so just go on and bang away if it makes you feel good.

If you want to work on a "better" Xenakis work, take a look at Mists, or the famous Herma. IMHO the best part of Evryali is the opening dance-like motif. I can't imagine how this opening relates to the rest of the work, but then again, I really don't care.
L.