I just saw this line in the new N55-based BMW 5-series review from Automobile Magazine:
"we sampled a 535i, powered by BMW’s new N55 single-turbo inline six-cylinder. The new engine is rated at the same 300 hp and 300 lb-ft of torque as the outgoing twin-turbo unit, but it doesn’t have quite the same punch at the low end."
Well, now we know why they still use the twin-turbo in the more sports-oriented model...
Go N54 !