As much as I clamoured over the tri-turbo, reaction in euroland was very lukewarm to the performance numbers. The M550d was definitely outshadowed by the M5, and rightfully so. The major complaint? Lack of performance in the lower end of the powerband (crazy, IMO, since we're talking diesel power here and three turbochargers - two small and one large, overlapping).
This will be a flagship only engine and its costs likely will be recuperated if sales of these ultra-'spensive boats go up because of the engine. It's one way to really be "unique". I support this, though it really is a shame that we'll never get these engines.
That M550d with the tri-turbo setup was good for the power mentioned (376hp; 545lb-ft), but had an (optimistic) consumption rating of 6.1L/100km. That's absurd, and very important for those in Europe.