I've used this product just as you mentioned, and it's performed about at the limit you set- 5 degrees. I've also used it by itself on cool summer nights when I needed just a light covering, when it was 65 or so. It probably outperforms another layer of clothes for a warmth to weight ratio, but the clothes will probably have a greater gain in lowering the comfort range. As a 10 oz. sleep system for the summer, it's OK. We used it this past fall on a trip in the Sierra where the average night was around 22 and we had brought 30 degree bags. I wore a fleece jacket and my wife used the Reactor. We both slept well. I found you need to roll the liner down when you get out, if you wish to return to it in any quick fashion. Once I learned that, it was no problem getting back into it.
I'll use it occasionally for a slight boost and by itself as a very lightweight sleep system. I will never use it to supplement more than 5-7 degrees, though. IMO, their claim is a little exaggerated.