You wont see a big improvement wrapping a stock header. It already has the stock heat shields which are essentially doing what the thermal wrap suppose to do. I see no point in wrapping a cast iron part, other that a turbo manifold. And yes it will cause hot spotting, accelerate warping, cracks and rust. If you have an aftermarket stainless steel or even ceramic header then wrapping your header will improve your performance mainly thru reducing heat soak. Cooler air for your air intake, help your cooling system out and longer life for rubber and plastic parts under your hood. You wont free up tons of hp, but every little bit helps and it's what car guys are always looking for. A tip for you is to wrap your header before installing and wear latex gloves if it's fiberglass.