I bought a gallon of Rustoleum professional flat black and mixed in some micro-spheres. Think super tiny glass bubbles with the consistency of flour. This makes a cheap, DIY version of Lizard Skin.
I painted the roof and floors with it, and after it had cured, I put down FatMat from the firewall to the tail lights and inside of the doors. I don't know how much better it is over not doing it, but with the carpet covering it, I haven't been able to notice any rattles, squeaks, drone, or other noises and I would say that it definitely tones down the inside of the car.
I think it also does a good job of stopping the heat.
ETA:
I stuck a piece to the roof about 2 years ago to see if it would stay in the Florida heat. So far it shows no sign of wanting to come off.