We have the exact same G37 Sport sedan (even same color!), but mine is a year older.
I have done a few HPDEs in my stock G37 and it is rock solid. It never did anything that surprised me and every corner was entered and exited knowing what to expect and it was very easy to steer with the throttle, when I wanted to. Miles of smiles
With the suspension mods you have done, you have added a lot of variables.
• Changed the front scrub radius by 35mm
• Changed the front track by 35mm and the rear by 45 mm. Since the track has been widened, there is more leverage on the suspension, so the effective wheel rate is softer/lower.
• Different spring rates and compression/rebound damping.
• Possibly different spring preload.
Items I would look into:
• Suspension travel. Is the rear hitting the bump stops when on the track? If it does, the wheel rate goes way up, and you loose traction.
• With a 275 rear tire, maybe -1.5 degrees is too much negative camber and not all of the tread is on contact with the road. As I recall, the G37 has a good rear camber curve and doesn't require much negative camber.
Bottom line:
If your car oversteers more than you want, you need to decrease the rear roll stiffness or increase the front roll stiffness.
Also, this may be of interest to you:
http://infinitig37.com/G37-BC-Coilover-Review.html