I came from a 2011 E63 and I can tell you the gearbox really is the worst part of the car. Often times it will not upshift or downshift and when it does it just seeems to respond sluggishly. I had the 6.3 engine so I cant really speak to the turbo engine on the current E63. I will say the M6GC engine is in a different league power wise to the 6.3 N/A engine. It revs just as freely but seems to put down much more than than the 50 or so rated horsepower difference.
14 M6GC- Black / Black - Dinan S1 (Dtronics, CAI, Exhaust, Coilovers) Sway bars
11 X5 3.5d - Titanium Silver - Black Interior