Well, here's my take.

Unless a metal heat shield has been made to fit around the turbo very closely and has been insulated on the underside to reflect the heat from the turbo, all you're doing is absorbing heat into the heat shield which will then radiate heat as well.

As an example, not that it necessarily requires any proving... there's a reason the heat shields on your exhaust manifold practically seal around the piping... and if you pull one off, you'll find fiberglass insulation pieces between the exhaust piping and the inner side of the shield.

I am not suggesting metal "roughly covering the turbo area" heat shields are totally ineffective, but I do think a lot of people don't understand the difference between the two approaches.

There's "keeping the heat IN" and there's "kinda keeping the heat around the turbo sort..."

Go touch the top of your metal heat shield at a stop light sometime