Just a late note on this tread. The website now lists the difference between the 550i and 550GT engine.
4.4-liter BMW TwinPower Turbo V-8, 32-valve 400-hp engine. Combines two turbochargers with variable valve control (Double-VANOS and Valvetronic) and high-precision direct injection
4.4-liter, 32-valve 445-hp V-8 engine with TwinPower Turbo technology, piezo direct fuel injection, 4 overhead camshafts, and Double-VANOS steplessly variable valve timing
I guess that I am confused. How many camshafts does the N63 have? Would the piezo direct fuel injection have that much effect?