Could you rig up an experiment with a piece of foam in glass jar and a vacuum pump or a vacuum cleaner to provide some suction? A piece of transparent hose for a water manometer and you should get some usable data.
Air pressure at sea level = 14.7psi = 407.3 inches of water
Air pressure at 8000ft = 10.9psi = 302 inches of water
You are only looking for differential pressure for 8000ft so your manometer working section can be 105 inches long (down the stairs maybe?)
My guess is that 8000ft pressure differential (normal cabin and hold pressure) is going to make little difference, as the amount of air in each cell of the foam is pretty miniscule in comparison to the amount of material in the cell walls.
Even if you went to full vacuum, the likelihood is the cells won't change a lot in size, although the foam may be slightly less resilient. If you were to go for large cell bubble wrap, or air pillows, in which the air volume is significantly greater for the cell wall thickness, then things may be a bit different.
Try the experiment with the foam at different temperatures, as the properties of the plastic will be affected quite significantly. You will need to have the foam in the hot or cold environment for several hours to ensure its not just the surface temperature that has changed.