I have a heated mat in my coop. I think it is the same one (22 inches) and is really an outdoor heated dog pad (waterproof with protected cord, etc.). However, I found it at a cheaper price at a website that specializes in Cozy Winters.
I have a small coop and I place the mat on the floor near the nest boxes (to lessen the likelihood that the eggs freeze.)
I'd say it increases the coop temperature anywhere from 5-10 degrees (nothing dramatic), which is enough to keep things above freezing on most cold NJ winter days. But last night, when it was 16 degrees outside, the coop went down to a low of 24 degrees.
It doesn't "throw" much heat, but it will keep water containers placed on it from freezing.
While using it, I also monitor humidity levels to make sure moisture in the air stays low. So far, so good.
This is my first winter with chickens and I am aware of all the arguments against adding any heat, but I'm also aware that it can be done safely and responsibly.
And my girls still choose to spend their days outside in the frigid cold, so I don't think that keeping nighttime temperatures above 32 degrees during freezing nights is "spoiling" them in any way.
Where are you located? Adult chickens only need heat below 0F. There are people in Alaska and Canada who don't heat their coops. Like dogs and cats, chickens grow a surprising amount of winter feathers. I had one attacked by a dog last week and we were sure she was dead from the amount of feathers we found, but we could hardly tell where they came from when we found here. Unless you live in the bitterest of cold climates, your chickens are fine.
You are right about getting cold hardy birds however, just going from warm to cold areas does not make for getting sick. Bacteria and virus make an animal (chicken, human etc) sick. A compromised immune system plus the addition of a virus can make individuals sick more often and more often a carrier of sickness to others around it but moving from hot to cold in and of itself does not do it. Think of athletes who exercise, getting their heart rates up all year long, being in hot and cold environments...
Shop the Coop has many nice items. However, I would def. try places like amazon (or just google) for cheaper prices. You might look up Sweeter Heater or flat panel heaters too. They don't "throw off" heat either, but just offer a warm spot (some wall mount them behind the roost). I built my own clay pot "heater" for my bantam coop. You can do a search on them here on BYC or pm me if you want pics.