It’s significant. Biggest effect is on efficacy of drug metabolism. There are many such studies.
One size does not fit all here owing to differences in expression of metabolising enzymes and drug transport mechanisms.
No, the Japanese are not aliens but they may require far less of a given drug than a westerner, or far more. This affects the likelihood to experience side effects for any given compound, and their severity.
It’s fairly ironic to make this change for Japan whilst elsewhere in the world with more racially mixed populations there has been a push in the opposite direction: a recognition that you cannot assume the results of a trial carried out on white male subjects will apply to those of African descent for example.
It’s not just drug treatments either. There are many aspects of medical care which have suffered from a lack of specificity and systemic bias.