I poked around a little and went here:
http://fastestlaps.com/tracks/willow...of_willow.html
These are the top track times *ever* on this track for each vehicle listed. Randy Probst is likely the "Golf Pro" at this track, knowing all the course nuances. His numerous records and the way he was slinging the 235 around were indications of this. Still, this list represents the best that *any* driver has gotten an M3 or Cayman around the track.
Maybe Probst has driven these other cars (M3, R8, Cayman) and simply didn't score the top time with them? Or perhaps he's never had a hand with these at Willow Springs. We can't tell from the data provided.
Regardless, Willos Springs is one of the major tracks in the Los Angeles area...there's a LOT of M3's, Caymans and R8's that have gone around that track before, so you can bet that the top times for each have had dozens of folks attempting to beat them.
Another impressive thing that the link above exhibits when you look at the horsepower column:
Except for the Subaru (309 hp) and Benz (360 hp), all the cars faster than the 235 generally have way more hp than the 235i. Most have 100hp or more over the 235.
The fact that this overweight, open diff, entry-level coupe is the fastest BMW ever around this course says a lot. They've done something very right with this engine/chassis combo. I imagine an LSD would scoot it a couple places higher.