I work for a german automotive company that has many global production locations (also supply systems for the e90), so I know there will always be differences in quality plant to plant. However, generalizing quality of a product based off geographical location of the plant is so narrow minded. There are so many factors that come into play, such as worker training, implementation of the quality system, worker morale, etc.
my company has plants in Germany, South Africa and the US (to name a few), and all three places have different quality. It's not the environment/location that determines the quality of the product, It's the level of implementation of the quality systems within the plant.
With Germany being the global headquarters, most of the advanced development and manufacturing techniques are investigated there. Most often, these standards are implemented in the local plants first, where it is easiest to integrate and audit the success of the changes. However, this is not always the case. A location, such as Germany, may not have the capacity to try new strategies, therefore South Africa (or another plant) may utilize the new techniques first, maybe because they are starting new production.
Unless you work for the specific company, you can't make such generalized statements that the German plant makes better cars than SA.
in BMW's case, since it seems SA has been a major build site for quite some time, I would not worry about quality. They most likely have installed the company's quality systems and processes. If there's a new China plant coming out, then I would worry - its not about the location, it's about how developed the location has become with the company standards.