The header wrap around the heater boxes does work very well. In addition to not loosing heat you will also reduce the temps. that radiat up to the heads when the car is running while sitting still. I did this to get more heat and yes it does work. The differance is luke warm heat to almost to hot to keep your hand over the outlet. In addition to this I also insulated any/all tubing all the way up to the cockpit. I've had people drive my car with these mods and they can't believe that my car has more heat than their beetle.
Will it rot out the boxes, if you drive your car in salt yes I'm sure it will. How many people drive their cars in the snow with salt on the roads, not many. Those heater boxes get real hot, I doubt you'll ever see them rust, if your worried about it get the boxes hot coated.
J-P