Insulating exaust

[GasSavers_Ryland]

as I understand it, the hotter you can keep your exaust, the more easly it will flow, is this correct?
I was thinking that insulating your exaust would benifit mileage, they do this to a point on race cars to help squeaze out more power, right? but that is mostly with a glass fiber fabric wrap, and that would soak up water, and dirt and loose affectiveness in weather, something that is not alwas an issue in racing.
so my idea was this, find an insulation that can handle heat, and find a way to protect it.
polyisocyanurate can handle a bit of heat, at least when I've put a torch to it, it does not melt or burn, so how about putting an inch thick layer (around R6-7?) and then cover it with epoxy renforced carbon fiber to protect it from the weather, and damage, making some points that connect to the metal exaust pipe of course, so if over time the foam turns in to dust, you still have this shell to hold in the heat, it seems like it would work better the tin heat sheilding that is used, I'm going to look around and see how hot I can get stuff like this, as my catolitic converters heat sheild is rusting away, and they are suposed to stay hot.
it also seems like a way to save a little weight, replacing the metal heat sheilding with high temp "plastic".