Cameron,
You are right; you must always use a pad or at least create some insulation between you and the ground. Otherwise the ratings mean nothing.
A tent will add probably about 10-15 degrees to the outside temp but this is a guess. But more then that it prevents the wind from becoming a factor.

I'll tell you from experience when it comes to keeping warm; body fat adds a lot. The leaner I am the more I am cold.

2 pads in winter is a good way to go, use closed cell foam on the bottom like a ridgerest.

Billy