It’s not just about keeping the heat off the realist of the engine bay - but keeping the heat in the turbo - (from the link I posted above)
A turbo blanket improves the performance of your turbocharger by keeping "the hot side hot".
In keeping the exhaust gases within the turbocharger hot, turbocharger efficiency is improved. As you may know, the hotter a gas is, the more expansive it is. Within a contained system of a specified size, the more expansive a gas is, the greater the pressure derived and thus, the greater the flow of gas to escape the containment. With this increased pressure and flow rate for a given engine RPM, the acceleration of the turbocharger's impeller is increased as compared to the same turbocharger with the engine at the same RPM but with cooler exhaust gases. This equates to faster spool up of the turbocharger, as well as greater attainable levels of boost. What a driver will experience with a turbo blanket is greater turbocharger responsiveness. The faster spool up of the turbocharger means less turbo lag and a more linear power curve.