i hate the term down sizing... it is more about right sizing, work with your fitter and see what is possible, AND what works for YOU each and every person has a tolerance to compression, for some people that tolerance is really good, for others its not. then consider how much ski time you are going to have is it 6 days or 60 in a season, most likely the 6 day skier is not going to tolerate or want the same compression as the 60 day skier, then consider that the jump in shell sizes is 10mm so if your foot in one size has a 15mm shell check then going down you get 5mm, fine for some not for others
with a note to what the top level athletes are doing remember that these guys have access to the factory for fitting, only a few shops have the same level of kit and know how to use it, next, if marcel gets a 25.5 shell and has the toe box punched out (common to do on race boots) then ground out (also common) and it doesn't work or he doesn't like it they just pick another pair of boots off the rack and do something different... if anyone has seen the pictures of the "lange" vandeer boot you will see in some shots the massive toe box punch which is then externally ground back to allow the boot to sit safely in the binding and still release, you can't pick that grind up down your local ski shop
back to "down sizing" pretty much anyone who wants a performance fit in their boots is technically "down sizing" as the boot you are in is most likely one down from where your feet measure on the brannock just to get something that isn't huge...... but length is only one dimension
i am sure that there a re a number of great skiers on this forum, but there aren't too many Marcels so why are people trying to get a tighter fit than he has????