the highest "high-end" mixte i own is only double butted chromoly. and you know what? it's heavier than my original high-ten one i found in the basement of a Harvard professor. And I mostly definitely love it that much more b/c of the "junk" status. I really suggest you just get the junkiest japanese mixte you can find/fit, and just go wild. put stickers on it. paint it with nail polish (mine looks like I ran over Captain Planet). mixtes are meant for tooling around, playing "let's pass the winded kid on the single-speed" while riding up hill. don't assume that the nicer tubing will make it worlds better. if you're after it for style points/lugs, try to find a peugeot, mercier, or earlier nishiki olympic royales (they had mildly elaborate headtube lugs).