First off, I'm confused about those JRB website figures. My Western Mountaineering MityLite has a loft of 3" with 12 oz. of 800 down, but its rated to only 40F. Now granted I've slept in it on a 35 degree night near water and I was fine (my head was cold since its not a mummy) but I can still tell you that its rating is nowhere near the 10-20 degree range the JRB numbers are quoting, though I'm assuming you can attribute this to a different type/weight of down.
I am a side sleeper as well so I enjoy the 62" shoulder/ 46" foot girth it provides. Also my WM is a 6'6" bag, which makes it large enough to accomadate my 6' synthetic Mountain Hardwear Lamina 20F bag inside of it, though it doesn't save any weight (combined the two are over 3 lbs...). I would think the Western Mountaineering MityLite would be a good outerbag for someone smaller or with a smaller bag.
If you use the Montbell as an outer bag, won't the elastic on the Montbell compress the inner bags' down and reduce its insulative properties? Just wondering, its a great idea and I like the idea of the elastic for extra movement, thats what made me buy my Western Mtn in the first place, the Mtn HW was just too constrictive.