I don't have a Nautilus, but an +35 Overbag. I couln't find a slot for it sooo. I have always used Down bags and have a +15 TNF Hot Tamale that has served me well, but in warmer climes it can be uncomfortable, like a 450 oven. The Wiggy bag looked like what I may need and I was pleased to find it was. I first used it on a trip to the Sierras fishing. The weather was mild to cold and the bage performed well. I alway use a liner in my bags, for the sweat effect, and I am sure it helped out. But, I didn't have to resort to the down for warmth. The only complaint was the chemical smell from the bag, it was brand new, so some odor was natural, given it was synthetic. After I returned home I washed it once in an earthfriendly odor free soap and rinsed it several times and the bag has no smell, to make sure I slept in it a few nights and the odor was gone. A plus from the washing was that the loft seemed higher.
I did use my down bag inside the overbag just to see the difference in warmth, blast furnace in a 28 degree night. Slept like a hotdog in a toasted bun, sweat liner was usefull. I am sure I could take the 2 down to - 20 or so with little additional insulation.
I was impressed and ordered the Superlite, between the three bags, should get me down to arctic conditions or Elk hunts in Wyo.
Oh yeah, I did order the biggest I could get in both bags, lots of room to toss and turn and my down has plenty of room to loft up to its max.

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Haven't had to use them, yet.

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Army down bags, Hunters bed rolls (couple of wool blankets and flannel sheets between canvas on a thin cotton mattress wrapped in a water proof canvas ground cloth) talk about low tech, I was young and could take it then, but the sleeping was good if you could stand the chattering of teeth.