In talking to a BMW tech I know this morning, he said that while they might share a lot of other suspension parts, sedans and wagons would have different rear springs, and that (as I've found) the part number application is very specific to your vehicle layout and options, which is why you generally need your VIN to order springs (OEM anyway). He also said that going heavier would just make for a harsher ride with no likely benefit of longevity. When his shop has to replace broken springs, they use OEM and never have them come back broken (at least not in as short a time as mine broke). I'm curious if I can get a part # off the springs that are in my car now, I bet they're not the right ones, because they were new in August '09. Or I need to dial back the driving style a bit, but that's no fun.
End result: the part number I got is consistent with the part number I got from Bavauto, so I'll be replacing with OEM and hopefully this set of springs will last a little longer.