Roger... a cozy wouldn't make much difference for you. :)
I did some tests in my kitchen today that relate to your question. I like hot coffee (not warm) and I use a GSI Cascadian cup, which doesn't have a lid and is single wall. I have a few of the Cascadian cups, so I lined up a test with them to see if it was worth it to make some sort of lid and cozy for it. I made a snug fitting relfectix cozy that ran around the outside wall, and I made a lid from a sour cream container that fit perfect (I even cut a drinking hole).
I boiled water and used an instant read thermometer every 5 minutes to see which offered the best bang for the buck at keeping my coffee hot over a 20 minute period. Conditions: starting temp 195°F, ambient temp 65°F with very light to non-existent breeze.
As you can see from my graph below, a lid+cozy on the mug worked best, but even just a lid makes a huge difference. In fact, after 10 minutes the cozy only added 7°F compared to the lid alone. After my test, I'm going to figure out how to make a good lid for it, but I won't bother with the neoprene cozy I was planning. Unfortunately the perfect fitting lid from the sour cream container is too flimsy and warps quickly when covering the hot water.
Bottom line, if you are rehydrating in a pot vs. dumping in to a bag, I don't think you probably need more than a lid. As it gets colder and windier, a cozy would start to make a bigger difference though.
This graph doesn't relate much to food in a freezer bag with no cozy vs. in a freezer bag with a cozy. I think in that situation, a cozy like the ones Sarah sells make a much bigger difference.