[Nagoyankee]

If I were to evaluate the synergy effect based on my conversations with the store clerks and restaurant employees who are Chinese here in Tokyo, I doubt if you can expect much.

Chinese speakers may know more kanji but that doesn't directly help you with Japanese grammar. Many of the Chinese around me seem to have a hard time conjugating Japanese verbs, choosing the correct particles, putting words in the right order, and pronouncing words. All of these are very important if you need to communicate in Japanese.

Your knowledge in Chinese will help you read written Japanese silently (as the kanji pronunciations are usually very different). Yet I honestly don't see any other advantages. As you said yourself, the two languages are simply not related to each other to begin with. It won't be like a Spanish speaker studying French or Italian, in which case you can expect unlimited synergy effects.