I've never read anything bad about Performance Gearing or Diffsonline, as you may have seen, there is also lots of recommendations on diffs over on bf.c track section. Both of those companies have long history of making reliable BMW diffs.
The only other diff builder who is a little less well known but extremely knowledgeable is Wanganstyle Global. I spoke with Haoming about an E82 diff and he had good insight on what is required to set up a reliable differential with the newer style 188L units that BMW installed on these cars.
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11 128i slicktop
13 WK2
19 Alltrack S
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