I have and use four of the nano's - they are fully rated and are excellent carabiners.

6mm is fine for prussick slings - that is typically about the size I use, but I would rather not use that size for a main rope. On the REI website the 6mm and 5 mm cords are listed as accessory cordage. Doesn't that say something?

Cavers, who do some of the most sophisticated ropework anywhere, in exploring deep and vertical caves, often use thinner ropes because of the weight savings involved. I wouldn't be surprised if stronger ropes become available in thin ropes, but there is a limit because smaller diameters are susceptible to more rapid and dangerous rope abrasion. Perhaps new materials will get around that limitation.