Another thought I had is that I read that the M57 engine is prone to cracking the exhaust manifold (it's welded stainless). I imagine a cracked exhaust manifold could reduce exhaust gas pressure to the turbo...

I'm sure you would have spotted this by now if it were the case. Symptoms such as a change in engine sound and a crap load of exhaust soot around the exhaust manifold would be obvious.

I took closer note of the boost in my 3.0d today, driving around town. Up steep inclines, the boost is noticeable by 2,000rpm, kicking harder by 2,500rpm and taking off like a two-stroke power-band by 3,000rpm.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)