According to the spec in the mag it's about 5" shorter in length, 4" wider and about 2" shorter in height than the G35 coupe. So there might be some weight saving due to the more compact dimension. The other thing might be the 3.2L vs 3.5L V6. Not sure if the smaller displacement engine is lighter or not. Then again with the TT come with the additional pipes, IC and the rest. Like you said it's hard to see where the 400lbs go.