While the GXP and the Impala share engines, their suspension setup is entirely different. The GXP gets Bilstein struts for improved handling while maintaining a relatively comfortable ride, and the biggest difference is that the front tires are 255's and the rears are 225's. This allows the car to corner better as the bigger front wheels lessen the effect of the front "plowing".

So they are NOT clones, as Rich accused :p .