Miyata used triple-butted tubing on production frames as far back as the mid '80s. I can't compare it to any other Japanese steel, but it feels like good stuff. But keep in mind, most Japanese bicycle producers, including the ones with a reputation for high-quality (Fuji, Nishiwata, Miyata, etc etc) produced a range of products. Frame and tube quality is generally directly correlated with the pricing of the bike. A Japanese bike that sold for $75 in 1985 was generally a solidly-built gaspipe frame. A Japanese frame that sold for $1,000 in 1985 was generally a solidly-built frame made from high-quality, lightweight tubing.

There is no doubt that from the mid or late '70s on, that the high-quality Japanese steel was at the same level of quality as Reynolds 531 or any other high-end Western tubing from the period that you care to name.