Very good...but destroys the exhaust if you use it in everyday or damp conditions.
I had it in my 4-2-1 manifold in my car and it rusted stainless steel in 2 years time.
Maybe a bunch of pot holders? welding blanket material? any other type of non flammable insulation should work (if thats what you want to do). There are gains to be had, but not that much.