This idea gets brought up on various boards every winter. It is a bad idea. First of all, for $900 (the cost of the MH suit) or maybe a bit more, you can buy the best -40C sleeping bag on the planet such as the Western Mountaineering Bison. If you need a -40C bag, buy one.
Second, a suit will not keep you as warm as a bag designed to be used at the same temperature. Separating your limbs eliminates the advantage of your body working as a unit to keep one space warm rather than five separate ones. I'm sure other people will have better scientific explanations than I am offering, but the principle is the same reason why mittens are warmer than gloves. If it was a good idea, high altitude mountaineers would do it, and they don't.
Third, the MH suit is not designed to be worn in warm weather; it is intended to be worn in extremely cold weather at high altitude. You would be swimming in your own sweat in this thing at anything near 15F if you tried to do anything in it other than maybe just sitting still. Will it keep you warmer than a 15F bag? Yes, but it would be useless for any other purpose at those temperatures and you have little or no way to control your venting of the heat you would be generating as you walk around in it.
If you want to see what Canadian winter campers who are out in -20C or colder weather wear, visit
www.wintertrekking.com. None of them are wearing anything like this and none are using it for a sleeping bag substitute.