Indeed,
The ratings on pianostreet are very limited (imo). After grade 8 it just goes straight to 8+ which would covers, for example, the immense gap between Beethoven's first and last sonata, which, technically, lie so far apart.
But to come back to this particular cycle of etudes, I still think these earlier versions of the Transcendental Studies (earlier ones are S. 136 right?) are comparable, as far as technical difficulty is concerned, with pieces like his Two Concert Etudes (Waldesrauschen and Gnomenreigen) or "Les jeux d'eaux à la Villa d'Este" from the third Années de Pelerinages. He wrote pieces which are yet much more easier than these.
Here is a list I suggest you take a look at. I played most of them so it's only my opinion of course, but it might help you:
- Liebesträume (Three Notturnos)
- Sposalizio and the Three Petrarch Sonnets (Années de Pelerinages: Italie)
- Invocation (Harmonies poétiques et religieuses
- Four Valses oubliées
- Transcription of "Spinning Song" from Flying Dutchman (by Wagner, but famous transcription)
Once again, these are my experiences and it might be very different for you. Also, keep in mind, if you really want to learn a piece, you always can.
Hope this helps!
BW,
Marijn