Fiddleback's experience is enough to show that there are some who like this setup! I guess my final advice to you would be to try it out (in the back yard or a trip close to the car where you can bail out if needed) to see if it works for you. As mentioned, the other option is to use the puffy jacket and pants to extend the range of a warmer (and therefore lighter and less expensive) sleeping bag. The cost of the jacket and pants therefore won't be wasted! Let us know if it works for you!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey