Agreed. It's pretty obvious. I wonder installing a rear underbody diffuser would help?

 

I also agree.

It's a lot of fun to get the S to over 100MPH but when you get there, you feel as if the rear is floating. My G35 sedan at 120MPH still feels in control but it's a heavier car.

Regardless, the S is so much fun to drive.

 

Wing.

Underbody diffusers work -- but only as part of a well designed aero package and only on cars that are slammed a lot lower than is practical on a street car.

Wing, on the other hand, works well even as a stand-alone element.

 

I don't go flying into a ditch when I hit 150+ but there is obvious rear lift above 120 IMO.

 

Maybe this is an AP1 vs AP2 thing - something with the suspension? Maybe tire brand or tire pressures, tire balance, alignment? Something else but at 100+ mph (which I hit nearly every morning on my way to work) the car feels VERY stable. Again this is all about "feel" but I notice no difference between 80 mph and 100 mph, even when I change lanes. There are so many factors that my cause one car have a different feel over another. I might suggest you take a look and see if you find any problems with your tires, suspension, or alignment if the car does not inspire confidence at speeds over 80 mph.

I've driven well over 110-120 on various tracks (Road Atlanta, Atlanta Motor Speedway, Roebling Road) So I'm really curious why you guys are feeling/saying the car is not stable over 80 mph. In my experience this would be total BS but maybe your car feels different or has some issue at speed that I don't have or even notice.