I have a 2008 Grand Prix GXP and a 2001 Grand Prix GTP. The 2001 is substantially faster because I have done a lot of work to it, but the 2008 is nice. the 2008 rides so much smoother and is so much more comfortable. I don't plan on modding it much because it already has plenty of power. I daily drive my 2008 and the 2001 sits in the garage. I bought the 2008 just after Christmas last year and since then I have changed the ac compressor because it was noisy, 2 motor mounts, and the harmonic balancer. I also changed 2 rear speakers because one of the monsoon speakers was blown. the only other thing I need to fix at this point is something in the steering because it is just a tiny bit loose sometimes.
I say all this as an example of stuff that wears out and/or breaks. My 2001 all had similar stuff break. I would not recommend running anything less than premium in both vehicles.
