There is another very important thing with BMW scenes. In addition to what has already been mentioned about an old scene containing only one car, there are two versions of the scene with two cars. The first original published in the forum by Mike Pan, download here:
https://blenderartists.org/forum/showthread.php?239480-2-7x-Cycles-benchmark-(Updated-BMW)
Then the blender.org published the same scene but using much more sample rate, so it takes more time to render. (Yes, this has made things very confusing for users)
So if you see short times out there, it is very likely that users are using original scene from this forum.