Hombre243, my point goes back to the OP who said he and his neighbor live 15 feet apart with stucco covered conventional walls.
My point was to construct two, let's say for discussion, 2' x 2' replicas of the two walls, space them 15' apart and shoot them with whatever HD ammo you prefer. That should answer the concern expressed by the OP.
Unfortunately, far to many people speculate about shotshell spread and penetration all the time without patterning their load against representative targets. Shooting paper targets at distance may define the spread but says nothing about realistic penetration.
Perfect example of not understanding is the use of flite control shotshells. Typically spread at 7 yards is the size of a quarter! Same as a slug. IMO, a recipe for a miss! Don't believe it, go test it.
Regards