I think you are on the right track.
Last year a friend of mine and I did a LOT of testing of different loads in our various 12 Ga. shotguns. We found that the shot cup of trap and skeet loads retained the shot to make it hit like a slug out to 15 to 25 feet (depending on brand, gun and load). Once the load hit a "wall" we made dozens of test "walls" of various materials and about 20" square, in his shop from scraps as he is a carpenter.
We found the shot cup full of shot would penetrate the first sheet of paneling, or even paneling over sheetrock or even double paneling over sheetrock (which his bedroom wall was made of). The plastic shot cup would then strip off the shot load when it hit the first layer of wall and the shot would spread and most of it would USUALLY penetrate all of the second wall. However only buck shots of different sizes would do much damage to a test wall set up behind the first wall.
Also we put up pumpkins and watermelons behind our test walls and found that the smallest shot 71/2 to 9 trap and skeet loads and bird loads would seldom penetrate the skin of the produce and usually none, not even one pellet, would go in more than 1/4".
We arrived at PMC 1 oz. of 71/2 trap and skeet loads at 1300 fps as our home defense loads -- cheap, powerful and LOW penetration. For some reason, these fast loads seemed to penetrate the produce the least after passing through both sides of the test walls. Sometimes these loads had only a few pellets that would exit the second side of the test wall. We also got some 11/8 oz of 9 shot at 1150 fps which seemed to penetrate more in nearly every case although our reasoning was that it would penetrate less. We still don't understand the physics of the two loads... why heavier, slower and smaller pellets seemed to penetrate more.