Originally Posted by Oulixes
The cost, quality, and professional installation are certainly serious considerations. The cost is difficult to guess right now, since the installation of an LSD on late issues of 3-series requires re-machining of the existing parts in order to install an aftermarket LSD. If machining is not done competently, the installation may lead to problems later on. Clearly, engineering skills of BMW would be preferred here, as being more reliable and allowing the modified car to stay under warranty.
I don't think that a supposed LSD from BMW Performance would involve any other things than swapping the entire final drive (pumpkin). It would be too hard and risky to do the actual diff replacement inside the pumpkin.