Glad my posts particulary outlining DH helped you make your decision... the GT is indeed an amazing car and one of the best all arounds... no its not an M3, not an X5 not a 750 or 550 ...but damn its close to all of them on so many fronts... i love it every time i drive it and i have many choices in keys to grab on the way out the door...
the 550 loaded as yours/mine are is indeed already an incredible handler and quick especially for its size... i will say Dinan does take it a step closer to an M3...right now my car can actually take an m3 straight aways and with the firmer springs ill play with them through any turns at any speed i love watching their faces as they see this monster sized car hang or pass them in the sweeps at 100+....it is only the brotherhood of BMW that gets me the thumbs up from them instead of the finger....
2012 GT 550ix Msport (feb12)(ok early march 12)
09 X5 4.8
12 X3 35i (the wife)
61 356 roadster