Hummel's cadenza is a historically significant addition to Mozart's K. 466, often favored for its adherence to the pianistic style of the early 19th century. It fits the concerto's mood perfectly while allowing for individual display.
Students must pay close attention to the delicate, historically informed ornamentation throughout the passage.
Comparable difficulty and character — good candidates for what to learn next.
Cadenza to Mozart's Piano Concerto No.20 (K.466) is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Cadenza to Mozart's Piano Concerto No.20 (K.466) lasts about 2 minutes.
Cadenza to Mozart's Piano Concerto No.20 (K.466) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Capriccio for Piano by Johann Nepomuk Hummel, Variations on a Theme of Corelli by Sergei Rachmaninoff, Cadenza to Mozart's Piano Concerto No.23, BV B 9 by Ferruccio Busoni — see the full similar-pieces list on this page.