If you want more stability, buy a hotshoe bracket, and another suction cup and pole, so it's fixed from the top and bottom.
This is probaly better for DV cameras (obviously), they are lighter, and there isn't much point in having a still camera in the car for trackdays.
Satanic, when you're testing, just use something with an equivalent weight, and drive your car pretty hard, straight line, braking and around corners.
The weight of the camera will surely surpass 3kg with the added g-forces, and you wouldn't want metal poles and a huge chunk of plastic floating around in your car on a track day.