"Please note that I'm not saying all monocultures are safe."
Sure, but you attributed safety and other "efficiencies" in Japan to "monoculture." You also wrote, "Multiculturalism means lowest common denominator."
Now you seem to be attributing safety in Japan not to monoculture but to /Japanese/ culture. Fair enough.
In that case, what would happen if 10 million Japanese moved to Bangladesh, making it more multicultural? Would it be a less "efficient" culture? Would it be anchored by the "lowest common denominator," or would the nation improve through the infusion of new cultural values? It seems doubtful that, after conducting this thought experiment, most people could honestly conclude that, "Multiculturalism means lowest common denominator." It likewise seems hard to imagine Singapore in its present state -- including efficiency-wise -- if all those ethnic Chinese has never migrated there.
The big context here is that you were defending Japan against criticism that it is a "closed off culture" and "too exclusive, too slow to change." That criticism is very well earned; for all its many virtues, there is a disturbing level of racism in Japan, against Koreans and Americans of black heritage (usually in military service there but not always, see http://www.time.com/time/magazine/article/0,9171,1101880815-...). There is also the matter of Japan's depressingly extended economic stagnation, going on 20 years old now, which is widely and credibly explained as being closely tied to an unwillingness to embrace new, often foreign values around business, corporate culture, risk taking, credit and competition.
The one time I visited Japan, for two weeks, I conjured many positive words to describe the country. "Efficient" was not one of them. The grueling, sit-at-your desk 14 hour workdays that are de rigueur in Japan are not particularly efficient. The make-work jobs everywhere also seemed inefficient, like the guy in a uniform bowing in apology non-stop to people driving by a construction detour in the road, or the full time ticket taker/thanker in the back of the bus who collected my payment stub. The many ritual greetings and apologies also did not seem efficient.
I have a hard time seeing how multiculturalism would not ENHANCE Japan's efficiency -- and economy.
Update:
"The swiss are also sometimes criticized for being unique in their ways and resistant to change. Culture is not the same as race or language."
In that case, Brooklyn is a monoculture of New Yorkers, or Americans, and your original distinction between it and Japan is rather meaningless.