Wow, you named three pieces in your post that I am working on, have worked on, or played part/most of within the last year.
The Mephisto Waltz is tons of fun....I say do it if you enjoy it. I stopped because I have to learn a bunch of other music for a compitition in 3 months.
Chopin Polonaise op 44 is awesome. It is IMO the most technically challenging of the polonaises (not counting Grande Polonaise Brilliante). I am working on this for the compitition, so if you start learning, we can give eachother advice.
The Rachmaninoff Rhapsody is TONS more difficult than it sounds. I would definitely say that it's more of a challenge than Mephisto Waltz. This is a Rach concerto, and the tempos on it can really go quickly. I am also working on this for the compitition. Some of the leaps, the octaves, and 16th note passages are insanely fast, but it is doable.
All three of these pieces are great, beneficial, and difficult.