I autocrossed my 128i convertible with sport suspension in a BMW Car Club of America
event in Marina, CA (near Monterrey) at an airport infield. This was a very well organized
event with a true coned course that challenged all participants as well as a figure eight that helped me understand the limits of the car.
I highly recommend membership (76,000 strong) the largest and best car club in the world, cheap to join and you get 'Roundel', hands down the best club magazine out there featuring Mike Miller in Tech Talk.
I had a blast running the ONE in that event, run flats and all, and found the car to be very competent and tossable. The Club has regional chapters and Autocross and car control clinics are mainstays and professionally run.