As for as weight goes, I've heard that True Temper had a bike built for them that came in at about 1 kilogram. I don't know which joining method or how it actually rode but that's how light you could go.
If you search on the C&V sub forum you might be able to find an old article that detailed a comparison of two bikes built identically but with different tube sets and paint. One of the tube sets was Tange Prestige. When test ridden by quite a few riders, most guessed wrong when asked which was the Tange bike, based on the ride qualities that the Tange tubing was supposed to have. That tells me that it's more about the builder than the tubes.