I agree with JCL... from my understanding all "35i" cars are run by the same N55 engine, but i suspect their slight difference in hp is due to a different tune on each model...
look into Dinan or Cobb tuning - they should give you a 15% gain with a quick flash of the ECU...

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'06 X5 4.4i| (08/06 built date) | Executive Edition | Sport Package
'08 X3 (wife's ride) - Sold
'05 325i - Sold
'03 325i - Sold
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