From my experience with hogs (about 800 pounds of famished pig), I must agree with you. I have seen a hog bust through a 2"x12" without much trouble. They are also great at destroying bramble patches, and taste better for the blackberries they eat when they do.

I'm sure they'll love the fresh grass, but I am also quite certain that if they decide to exit the structure, what I saw won't hold them back much.