Don't waste your time or money. Wrap for headers does a lot more to keep exhaust temperatures high, but less for keeping your engine bay cool. You're better off with a heat shield between your air intake and your exhaust (like the TDR heat shield).
Also, the higher exhaust temps WILL be felt in your floorboards and transmission tunnels (where the wrap ends, and the higher temperatures radiate from the pipe), unless you add more insulation under your carpet. I learned this the hard way with a Porsche 924 (that started overheating post header wrap). The car was an oven.
Racers use high exhaust gas as a method of reducing the viscosity of the exhaust, and thus lowering resistance in the system. This is a big reason why you see wrap on race cars.