Drove the GT a few days ago. Loved it.. what a blast. The noise is awesome. I did not notice any appreciable detriment related to the live rear axle.
That said, the overall package is disappointing (though, makes sense considering the price). For me I'd rather just pay more for an upscale interior, intelligently designed greenhouse, and arguably more understated exterior design. I am also strongly concerned about reliability and resale value. Here in Calgary, American cars have unbelievably bad resale values. Horrible, in fact; becoming more noticeable with each passing year.
///M Power - 2011.75 LMB M3 6MT (fuelly)