I had no problems with the Engine Blanket during the winter season. It is still on the engine at this time, and temps are still manageable by pulling back a corner of the radiator block, and/or climate controls.
As for increasing the overall FE, it certainly didn't make much difference this winter. Worst winter of the 3 years that I have driven the Insight. It does help retain some of the heat in the coolant. After a 10 hour workday the coolant would be in the 25-30F above ambient. At about 4 miles out I would be seeing 195F coolant temps. Then start burping the climate control to increase the cabin temps. And that's with easy does it hypermiling.
Combined with the WAI pulling from inside the heat shield on the cat, I noticed a decrease in the frequency of the NOx purge events. I think the little ICE was much happier.