When I first started selling cars (Nissans) in 1986 we had Japanese and American-built Sentras (Smyrna, GA.). No doubt about it, the American ones were inferior. It was most obvious, even visibly so as the rubber exterior bits like windshield seals, black bumper covers, etc. all began oxidizing almost immediately on the US-built, and not at all on Japanese. There were several other noticeable differences. My next stop, Honda, same thing. The Japanese Accords made of better materials, also better fit and finish. We had plenty of customers refuse delivery based on assembly point. The only similar BMW comparisons that could be drawn were with the SA built versus German E46 sedans. They were very close, I couldn't tell a difference, yet again, we had buyers refuse SA-built vehicles.
For me the point to be gleaned is that the jobber/supplier issue is more salient than the assembler issue.