I don't have the Japanese Castlevania 3, but I do have the CD soundtrack to the first 3 NES Castlevanias (I actually wrote a review of that soundtrack. Maybe I should post it...). If that soundtrack is any indication of how the Japanese version sounds (Castlevania 1 & 2 sound just about the same) the difference is noticible, but not overly improved, except for Vampire Killer, which has a lot more musical activity. Overall, I'd say the sound is very Casio-keyboard-like. Kurt Kalata has the Japanese version and says that his converter DOESN'T give him the extra sound.