Just sat down on my couch after playing that at the tempo you suggested. Not bad and not hard. I too have superb octave technique. I have not played the full op.53 Beethoven but I've played the prestissimo section and can get the glissando with articulated octaves that are hit individually at speed.
I will give you this, if that smallittle tiny section was continued on and on for measures and measures and pages and pages. It would be challenging, but not impossible. You speak of these as if they are the Everest of octaves. You couldn't be more wrong, you also speak of the 6th rhapsody the same way.
Mazeppa
Liszt b minor sonata
Quite a lot of Schubert has painfully awkward and fast octaves
Allegro barbaro Alkan
Alkan op.39 no.11
The list goes on and on, octaves are doable with the right technique and do not need a super virtuoso to pull them off. I defenitely do not have super virtuoso technique but octaves do come extremely easily and I've never had a problem with them, ever. I can play the ossia finale to rach 3 with no problem, and I like to have fun on Fridays by turning most of my pieces into Botha hands in octaves. Yet I still lack in a lot of areas with my technique. I am working on that, but still. Octaves aren't bad if you are good at them.
It's a stupid statement but it's true.
Go listen to zimmerman play this section, and Horowitz and Rubinstein. It's not played at a ridiculous tempo, it's played at a very musical tempo. And after all, isn't this music?
I didn't say they are an everest. I said they are both fast and difficult. Above all, because they must be shaped (unless the pianist has slack musical standards that allow mere execution of the notes to suffice). Zimmerman plays it with two hands. Rubinstein probably does too, knowing his attitudes (not to mention the effortless and truly rapid execution). Horowitz (much as he's my favourite overall pianist) makes a complete pig's ear of the passage musically, with nothing like a seamless legato line. You can literally hear the breaks and the excessive heaviness, as he overworks to get the notes knocked out at speed. It's interesting that every example you gave there from pieces is of cases where this type of playing actually works- rather than instances that demand significantly phrased octaves to be played at extreme speeds. If this were such a case, it wouldn't be difficult with 15. This passage is actually rather rare, in terms of the specific musical difficulties.
Also, you didn't reply to any of my specifics. Are you still sincerely telling us you play even the last statement in the liszt rhapsody at speeds notably in advance of eight repetitions per second? Did you stop to try that metronome mark?
Here's Katsaris, showing his consumate virtuosity by shaping the octaves and also extolling the importance of physical legato in making that possible.
See 3:40. Note how the student achieves fair speed, but nothing like the sense of seamless line Katsaris makes- even with legato fingering. When you take musical issues into account, this is not remotely basic to pull off, even with a legato connection. Without, it may even be outright impossible to achieve fluidity. Look at youtube films of various accomplished pianists playing this and you'll find that just about anyone who plays it at such speed and with such lightness visibly use two hands. That's no small feat that Katsaris is pulling off, regardless of how casually it's done!
Here's Horowitz making a hash of it by failing to keep either smoothness or lightness (about 27 seconds in):
The notes sound punched out individually (yet rather messily due to the reliance on pedal), not smooth. There's no hint of a legato illusion to be heard and it's not actually very fast (despite the overwhelmingly audible effort going into the execution).
Sorry, but as I said before, words are cheap. If you feel you'd meet either my musical standards or those of Katsaris, you should upload a recording of what you say is not challenging to do. Then we'll judge how much of a phrase comes across with 15. I'd also be very interested in hearing evidence that you actually reach the speed you claimed would be slow for the 6th rhapsody, otherwise I'll take it to have been bravado.