Heat shield, but be prepared to be taken to the dog house by others members who will question your logic.. I installed the PTP blanket on top of my heat shield, and I can tell you the heat reduction in the engine bay has been significant.,which was my goal.(.not squeezing out some insufficient amount of horsepower, since I'm not a racer.). The reason I know, is that every time I pull into the garage, I open the hood to let it ventilate the heat out.
I believe these turbo blankets, Sila or PTP, do not fully encapsulate the hot side of the turbo. Some underside on the turbo is not covered thus not allowing for the full benefit of a fully covered turbo, which we all know increases efficiency of the unit.
I know that turbo housing units are pretty robust, but I would venture to say that a turbo housing unit that is hot on one side and cooler on the underside COULD lead to metal fatigue, in the very long run. Who knows?? Any thoughts, or discussion??