http://www.lddb.com/search.php?search=thx+1138&sort=title
The LaserDisc Database lists 2 JP laserdiscs for THX 1138. The 08JL-61162 (1986) is P&S (fullframed pan & scan) and the NJWL-11162 (1995) is WS (letterboxed widescreen).
The quality issue, as well as the censorship issue, were raised on the 1st post page of this thread. (I don't think anyone established anything conclusive thereafter.)