I think the challenge is that none of the really powerful BMWs are both manual and AWD. That means the "easy" options are using any of the powerful V8/10/12 motors but settling for RWD (as with the E39 M5 or 850Ci) or using the 3/530/35xi package (which gets you 255-300hp).

I don't know if the E53 3.0i gearbox will work with a more powerful V8, but I've never heard of it being done.

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2002 X5 3.0i