(Thanks for the responses Ron). Car is still at dealership. The tech noted that cooling fan was failing (see my earlier update re: overheating), and that has been replaced, but problem is persisting according to SA. Initially they were hoping the overheating was the problem, as tranny fluid is cooled via radiator.
They've opened a tech matter with BMW directly, and are trying to ascertain the problem, so it may be a software glitch, or worse, a manufacturing problem. I am not sure if the tech matter is opened straight with Germany (where they make the tranny) or with USA directly, but I'll try to figure that out today. No codes appear during testing, but there obviously is a problem.
My car has just at 10,000 miles on it, and the problem began in December (reached 10k by January). After three weeks of trying to schedule a service appointment to no avail, I finally drove it in and explained the problem. So my guess is the problem started happening around the same mileage you reference (around 9k or just over that).
I have a loaner 325xi in the meantime, which is fine to drive as it does not have the acceleration issue.
More updates later, as I expect to hear back from the SA today.