any chopin etude difficulty poll that yields a result other than 10/2 is simply invalid.
a piece like the 25/6, i have no doubt the legendary sightreaders like richter, libetta, katsaris etc could play it up to speed at first sight. 10/2? not a chance
I actually would not judge on "sighreadability" as the main factor. For example, the 25/11 might be much harder to read, but in fact, it is pretty much straight forward etude, ones brain and fingers "remember" what to do.
The bottom line, any of the Chopin etudes might be whether "comfortable", or basically unplayable, depending on individual strenghts of each individual performer.
If one had a good professional training since childhood and has big enough hands (for such etudes as 10/1 and 11, 25/8 and 10) none of them should give particular troubles.
As for 25/6... once again, if as a kid you have learnt how to play double notes, you should have no particular difficulties there.
The 10/2 indeed stands out as THE MOST difficult one. But its difficulty is in patience of a few years of EVERYDAY 1 hour routine, which BTW, does not guarantee you can play it even then.
Let's put it like this--if you are able to play 10/2 with great control, very quitely, absolutely evenly all the way through, then MOST LIKELY the 25/6 will be piece of cake for you. The other way around might not neccessarily be the case.
But the REAL KILLER is 10/2 right after 10/1! Besides many hours of practicing, the trick is in absolute mental calculation. The good example of lack of one is Lisitza's live recroding.
Having said that, I have one particular etude I never had patience for--25/4. I just never could force myself to sitdown and for sometime work on LH alone to make it 100% accurate. So... I just don't play it in public--big deal.