Although I do not have a 100% definitive answer, I believe that there must be distinct differences between the foam used in polishing pads and the foam used to pack shipping cartons. My assumption would be that shipping foam is inexpensive, and nothing more than a filler to keep things from shifting around. Polishing/Buffing/Finishing foam must have certain physical properties. Compounding pads must be firm, porous and able to cut. Finishing pads have no cut, but provide an excellent shine when used properly.
My opinion is that you should not use this packing foam on ANY car. Instead, if you are truely curious, find a piece of scratched or faded metal, and polish that. Metal drums, washer/dryer, or old Coleman coolers are a couple of things that come to mind to test this foam on. But I would not start out by trying on a car.