>Do you think Japan and Japanese people would have a higher or lower quality of life if they had accepted unlimited sub-Saharan Africa immigration?

Higher. After a while.

Signed, someone whose ancestors thought the Irish and Italians were subhuman degenerate invaders.

Now that I think of it, I still have living relatives who think the pope is the antichrist and people who follow him are degenerate subhumans.

So your answer is "higher" with a moral vignette of dubious veracity and applicability.