The Roslavets are thirded by me, but those are damn near impossible. Hamelin said that he had to completely renotate them onto two staves, and that they were some of the most difficult pieces he has ever played.
The Kapustin Op. 40 etudes have been done to death, but they are still very good works. I will, however, recommend his Etudes in Different Intervals, Op. 68 and his Three Etudes, Op. 67, neither of which are performed very often.