This past 2nd Annual Florida Hang, it was down to 28*(super cold for Florida). I was using a wm pad and my back was not cold at all. No wind either and my sleeping bag around the hammy like a peapod.
My guess, just like other items, it depends on the sleeper. The same pad may be 30's -40's for 1 and colder for others. Good luck in finding the right temp for you.