Ok, basically answered my own question, but I thought I'd share.
First, here is a detailed review of the Wei-Tech 5.1, including mention of a few construction issues:
http://www.airsplat.com/hicapa.htm
Then I took photos similar to the ones found in the above review to detail the difference. The problems mentioned in the review regarding the Wie-T are not found in the TM. Inner barrel fit is snug; hammer is well crafted, cast and finished; the dual safety is properly balanced and works correctly from either side.
- note the metal beaver tail rear safety, metal dual safety, hammer, lower receiver, bomars. Theres lots of metal even on the stock TM. Metal mag release, slide catch, user mountable lower rail and the guide rod, along with much of the internal structure make for a very solid feel. Still, only half the slide action is metal on a stock TM Hi Capa and it sounds that way - half metal, though chunky nonetheless.
From the clues here and with other cloned guns, it seems the Japanese made items gives a higher degree of precision. Is more metal worth the trade-off of less precise manufacturing specs? I suppose that varies by user.