I have a 10" Kicker hideaway sub under my rear seat. I have never heard an OEM GM kicker sub kit, so I can't compare, but for the size of the hideaway sub, it is fairly impressive. It adds enough bass to be enjoyable without being all distorted.
I tied into the factory Bose wiring behind the backseat and mounted the volume control under the dash for easy adjusting, even though most of the time, I don't mess with it.