Was Yasuhisa Watanabe ever actually part of SuperSweep? The only confirmation I can find is from his Wikipedia page, and it's been there since the article's creation.
His last.fm profile doesn't mention SuperSweep:
http://www.lastfm.jp/music/%E6%B8%A1...81%AD%E4%B9%85
And from the back scan of the first SuperSweep release that contains his music (SRIN-1007), he's not linked explicitly with SuperSweep, and Studio Dual Moon is already in existence:
http://vgmdb.net/db/covers.php?do=view&cover=8441