"They recommend against it due to over expansion and over penetration."

Over expansion and over penetration sounds good to me! If it was one or the other, I would need to make a decision. I would be happy to take both.
As has been noted above, the idea that a couple of inches of extra barrel length in a .38 Spec will cause a round that just meets FBI minimum penetration in a 2 in barrel will now go through anything encountered while making too big a hole seems incredible. The new "worlds most powerful handgun"! That could save my day.

Rick