The stock sound system on my GTP is fine. Given how loud the car is (it's ~13.5 years old and has typical ****ty UAW interior that's as quiet as a freight train), I think dumping cash into a high end stereo on the car is a waste.
Back in the day when I was into car stereos (back in the face-off CD player days), if it was >100W it was typically a POS. They're spouting peak power. A GOOD 40W amp will demolish a low-end 1000W amp in quality and performance.
Like anything, you get what you pay for.
I'd leave it stock - but that's your choice.
