In reply to Sealwife:
I suppose all the usual concerns still apply - shoes will stretch to the shape of your foot (good), and increase in size a little (less good) and therefore a larger shoe will stretch less because you're asking it to mould to the same foot (which hasn't got any bigger just because your shoe did).
I've always been a massive downsizer myself - as a 43 street I wear Scarpa Stix in 40.5, Genius in 40, Vapour V in 41, Instinct Lace in 41 e.t.c. I ended up with a 41 Arpia.
I did, however, recently watch a vid on youtube with magnus mitbo (of all people) sat down with Heinz Mariarcher (who designed all of these) for about 20 mins speaking about shoes. One of the things he says is that lots of these shoes weren't designed to be downsized that far - his words (roughly) "you can get precision out of any shoe if you downsize far enough, but if you end up stretching the toe rand to bulge you will roll off of footholds due to the angle of pressure, or if you downsize a shoe that wasn't meant to be that tight you will waste the active randing and complexity of construction that's designed to provide grip". In reply, magnus is like "Yeah, so I go 2-2.5 sizes down in these (furia air)", and heinz is aghasp - one or one and a half only or your smearing will go shitty.
Anyway, perhaps this is me becoming a reformed downsizer.