The question isn't about cultures and the ability for Mexico to produce quality BMW's.
The question is how long will it take for the BMW Mexico plant to get their process and product quality up-to-snuff.
The best regional option on the quality side would have been to produce it in Spartanburg, not because it's US and US workers are that much better, but the facility's processes and supplier base are well established to work with BMW. The communication is working well enough to produce some of the most expensive and popular vehicles in BMW's lineup (X5/X6M). The E70 in general has been a relative gem regarding Quality, especially the later year models (09+). Having this production capability cascade down to the next 3 series would not be a bad thing...
Although many suppliers will be shared between the two plants, there are some components/systems that will be newly sourced out of Mexico (suppliers with facilities in both US and Mexico).
All-in-all, the people of Mexico are capable of producing quality BMW vehicles. The quality standards that will be upheld will depend on BMW Corporate's ability to manage the new facility and ramp up the production location. Dealing with different cultures (especially new ones) is a challenge, and the new facility will definitely be going thru some growing pains.