This is a classical work originally scored for choir and orchestra, though often adapted for solo piano to facilitate study or intimate performance. It exhibits the structural clarity and homophonic textures typical of late 18th-century liturgical composition.
The difficulty lies in transcribing the orchestral density effectively, particularly regarding the voicing of chords and orchestral textures.
Comparable difficulty and character — good candidates for what to learn next.
11th Mass is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 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 11th Mass lasts about 25 minutes.
11th Mass 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 Boléro by Maurice Ravel, Prelude in C-sharp minor, No. 2 by Sergei Rachmaninoff, Prelude in G minor, No. 5 by Sergei Rachmaninoff — see the full similar-pieces list on this page.