How much of this difference can actually be attributed to the followers attempting to obey religious precepts

Religion is more than obeying general precepts. During the time my Catholic grandmother was in school she wanted to read some book. Before reading it she asked her priest to allow her to read it because it was on the Catholic census. Following the religion seriously and not reading anything that's on the census has an effect that goes beyond the general precepts.

A lot of Buddhists are vegetarians. A lot of Buddhists mediate. Those practices have effects.

and how much is simply floating in the sea of cultural memes in the parts of the world where Buddhism and Christianity respectively happen to be common? Religion isn't more than a bunch of cultural memes packed together into a packet.

Your question assumes that people in Japan can be either "Christians" or "Buddhists" but can't be both. Even when the Chrisitans in Malta pray to Allah you can't be Muslim and a Christian at the same time. There no similar problem with being a Zen Buddhist and being Christian at the same time.

Would you expect practicing Christians in Japan, Korea, China, or India (and who are ethnically Japanese, Korean, etc.) behave more like your model of "Buddhists" or "Christians"?

I think that there a correlation but I'm not sure about the extend to which Far East Christians resemble Western Christians. Making a decision to convert to Christianity when you live in China has a lot of apsects that don't exist when someone who lives in a Christian town simply decides to stay Christian.