I don't doubt for a minute that the M3's coming out of China are of better build quality, much as they would be out of any NEW factory, but I'm willing to bet they're not THAT MUCH better. I'm reminded of a very recent "employee engagement and satisfaction" survey done annually by my previous employer. Oddly, the employee satisfaction scores from the China where all MUCH, MUCH higher than that of the ones from North America and Europe. Turns out after looking into the matter more closely, the employees in China all had to attend mandatory training on how to fill out the survey each year. While they were not explicitly told how to answer, they were guided on what their answers meant to the company, and how detrimental certain answers could be. They were also told that if they scored something low, it was their responsibility to back it up with explanations and suggestions for improvements (which we all know might result in more work). From speaking to my Chinese counterparts at the time, this type of thing is normal there.
One last thing, when manufacturing defects occur, we like to blame the assembly line workers who messed it up. In reality the problem is a lack of good management setting the proper expectations and quality controls, allowing defects to go out the door. Its all part of a system that starts with bean-counters deciding how much each car should cost to make, and how many cars should go out per week, and then everyone scrambling to hit those numbers rather than focus on making a quality product. We incentivize quantity over quality. Everyone's bonuses are always based on "shipping XXXX units", rather than ".005 defects per unit"
FWIW, my M3 has been pretty rock solid in the 11 months I've had it. Only one service center visit, and that was to install Homelink. Yes, I'm now getting a couple new rattles, so we'll see where that goes.