The "edge effect" sounds like a tough problem to handle. Basically, as I see it, you could lose heat in three areas. The first is right where you are measuring (the bottom). The second is on the sides and the third is on top (but through the mat first). The same is true out in the field. The heat can transfer into the pad, then out to the sides, or back up to the top in the section of your pad that isn't covered by a sleeping bag/quilt. Solving the problem with the sides of the mat seems easy -- just let it happen. If a pad loses some heat to the sides during testing, then it will lose some heat in the real world (unless the camper insulates the edges). Solving the loss to the top of the mat sounds harder. In the real world, part of the sleeping bag/quilt covers it up. But depending on the shape of the mat and the sleeping bag/quilt, some of it is uncovered. To get a good estimation, you might want to measure things with various coverings. Start by laying a good bag over the rest of the mat, then cover only part of it, then cover none of it.
It would be interesting to see how mat combinations work under these conditions as well. As has been reported by many people, if you have an inflatable and a closed cell mat, you are better off putting the closed cell on top. Without the edge effect, I would assume that it would make no difference. I would be curious as to much of the loss is due to the sides of the mat, as opposed to the uncovered top.