I played mazeppa using 24 24. You have to, I refuse to accept any argument against it. It is possible to do, and it isn't that hard once you get the technique sorted.
You havw to use 24 to stop the piece going to fast, and they must not be legato.
They etude is an etude in arm movement, the whole key to playing this etude is finding the correct movements, you have to decide how to get from each chord, and what movements are most efficient.
The 3rds must be totally clear, and 24 is the only possible way to play fast and non legato.
the 24 forces you to use the arms, using normal 3rds fingererings uses the fingers istead of the arm, and you loose power, but gain speed.
Also, the theme must be the main feature of the music. Thats like the horse, and the 3rds are like the dust after each step. Have you ever watched a horse run in slow motion? There is a moment when all the hooves are off the ground...this is what it should be like when you have just played the last 3rd...your hands are totally free, and have to find the next chord. You have to get the movements perfectly co-ordinated.
The 3rds must cresendo from the 1st to the last perfectly. If you use legato fingerings, you will bump on the 1st and the 3rd 3rd. and you will get a nasty phrasing.
I found the shape required for the 3rds was important. At first I just played them normally, but that didn't work, it was to weak. I found the you have to make a wedge shape, and keep that shape. And the big key is to keep the arms moving, so the hands do NOT actually cross, they just smoothly pass. You have to decide how to get from the 3rds to the chords. Thats the hard bit.
I don't think Liszt wanted this piece to be fast. But you can create the illusion of it being fast by making sure EVERY note is heard, and 24 24 makes this possible. If you want to make the piece sound seriously fast, keep the same speed from start to finish. 1st theme semiquavers, 2nd variation triplets, 3rd 2 quavers, 4th (and hardest) theme 1 quaver. If you keep EXACTLY the same speed, it sounds so fast, and you gain so much momentum.
This piece needs lots and lots of slow practice, but perfect slow practice. It's very hard. If you practice all the movements perfectly, you won't have to worry about speed.