I don't recall where, but there was something I wrote (I believe by Ed Headrick) about the creation of the 150 class, and why it's used in Japan. It's actually on the law books there (if I remember correctly) that the weight of a flying disc under a certain diameter cannot exceed 150g due to a lot of their disc golf courses being in public parks that see a good deal of pedestrian traffic.
(edit) Here's the link:
All that said, I doubt anyone's checking. But, that said, Japan is a country that cares very much about honor and respect, and I would bet money you'd lose both in the eyes of any Japanese people you played with if you played with heavy discs.