-15 is chilly!
I have not used the Wiggys overbag, but would be skeptical of the 40 degree added warmth claim. As Ross mentioned, the design requirements for overbags necessitate a heavy item, and so most are at least 2lbs. They do work well when the temperatures never get above freezing and internal moisture is a concern with down bags. This is generally what they are for, and is why most are synthetic and many have waterproof shells (think of them as insulated bivy sacks).
If you really will be in -15 temperatures you might have many days where it does not get above freezing and a synthetic overbag may be a good policy against loft deflation in your down bag. However, paring you bag with a rectangular down bag or down overbag will be a lighter option and pack much easier.
Their lightest is about 22oz and claims to add 25 degrees. It is also pricey and uses down, which defeats the purpose of an overbag IMO. Anyway, check out their offerings for a good selection.
Mountain Equipment Co-op also offers two, the Penguin and Emperor with the former being under $100. Both are heavy.
Integral Designs has the Andromeda, which is again heavy but uses good materials for the purpose.
There is also a very basic overbag made by Cocoon for $100. I have not found a listed weight, but its design suggests under two pounds (no fill on the bottom and no hood).
Lastly, if you are trying to boost bag warmth the easiest way is to use a very warm sleeping pad. A down air mattress (I do not recommend Exped, mine has had numerous leaks) can give you an extra 10 degrees. Also, add some down booties and a down jacket (or synthetic). So long as you have enough room in your bag, wearing insulated clothing really helps and makes getting out of your bag on a sub-zero morning MUCH nicer.