LG Japan has released new firmware version A208.
It fixed the problem with some discs not burning at their supposedly rated speed, mostly the crappy brands - i.e. "Vakoss recordable" (blank silver top) burns now at 8x but I havent try any other crappy media yet.

Selfextracting package containing firmware update .exe and japanese instructions is available here:
http://www2.melcoinc.co.jp/pub/hd/dvsm388fb_fwa208.exe