Well... Here are some of the world`s hardest pieces -
Beethoven - Hammerklavier Sonata
Balakirev - Islamey
Liszt - Sonata in B Minor
Rachmaninoff - Rach 3, Sonata #2
Prokofiev - Sonatas and concerti
Scriabin - He wrote some insanely difficult things...
Sorabji - ANYTHING!
You could always give Rach 3 a shot! Rach 3 is absolutely my favorite piece in the world. If that doesn`t give you a problem, then you should be concertizing. I`ve never played Diabolical Suggestion, so I don`t know how difficult it is in comparison to these pieces, but hey, there`s no problem with giving yourself a real challenge!