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More than likely the turbos are made by Borg Warner or Mitsubishi.
Torque handling is hard to say, one would need to either push the limit or find out from BMW.
Turbo limitations, good question. AR design might know the turbine housing A/R's. Turbos are both oil and water cooled.
A different turbo manifold design is quite unlikely due to space limitations.
Tuners primarily increase boost and change air/fuel ratios. If you are very concerned with the details on how it works, you are best buying a book on EFI tuning. I think buying a tune from a reputable company like ESS or AA is going to be much easier for you.
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