So I think I read through all of that, and I've known about this repair for years from the BMW forums, but I still have never seen any factory directive about its use to reinforce cars that didn't come with the reinforcement from the factory. Near as I can tell, only M3s came with it from the factory. Not the 325i, not the 328i. Those reinforcement plates are not listed as superceded parts for anything either, since they are additional parts, not replacement parts. Seems to me that someone just "figured out" that the M3 had reinforcement that the non-Ms didn't have, so it became the de-facto way to repair this problem.

So quickly, did any factory documentation ever turn up?