The primary purpose of the Dynamat-like products is to turn vibration into heat.
Actually, very little heat is generated, but they are effective at reducing vibration-induced sounds. Their reflective shiny surface is probably somewhat of a radiant barrier, but perversely is designed to be mounted in the interior of the car, where it presumably is effective in reflecting heat back to the occupants, instead of preventing it from getting into the cockpit in the first place. The darker variants from other manufacturers (a dull black instead of the shiny surface) are better at absorbing radiant heat.
Real heat insulation is the province of dedicated products designed to do just that. For example, Aeroquip Firesleeve is a great tool for keeping heat off shifter cables. The turbo blankets that DEI markets are great for keeping heat in the turbine. And while both Firesleeve and the turbo blankets have a very small effect on sound, their primary purpose, and effectiveness, is heat insulation.
Products like Lizard Skin are also largely dampening products that may have a relatively small effect in terms of stopping most kinds of heat transfer. A quick look at their webpage shows that even their claims for their LizardSkin CI product which includes "ceramic insulation particles" are without any information about the kind of heat they are supposed to reject. They are probably better at rejecting conductive heat, but like the mat products, are mostly a way to reduce panel-induced vibration noise.
There are different products that are optimized for different kinds of heat. For example, radiant heat barriers are often very reflective, and thin, like the popular gold foil seen in many of our cars. But these are relatively less effective against conduction or convective heat. Products like Firesleeve are not as good at rejecting radiant heat. That's why you sometimes need a combination of tools.
That's why it isn't as simple as just putting "insulation" on something- you really have to know what kind(s) of heat you are trying to manage. An example: My shifter cables are protected with several layers of insulation, with different kinds at different places, depending on the kind of heat the cables see at that point. There are places where the cables are exposed with no insulation at all, and a couple of places where they are encased in a combination of Firesleeve (to protect against mostly against convection and conduction) and a wrap of aluminized tape designed to reflect radiant heat. That combo seems to work really well, and my car shifts perfectly even in a heat soak situation. As many builders here know, at least one car has gone through several sets of cables until the right mix of routing and insulation was figured out to stop heat from melting the cable sleeves.
And finally, in my car, I have a fair amount of different kinds of insulation, each designed for a different kind of heat source. But I am finding, to my dismay, that a key part of a quiet SLC is probably more a function of sealing, rather than a focus on vibration reduction from tools like Dynamat. Allan U has done some good work in this area, fabricating sheetmetal seals between the body and chassis, as well as closing off the side tunnels at the front with similar aluminum and fiberglass seals there. These seem to make a noticeable difference.
Based on experience with my car and others, I'd say the most important thing is to protect vital parts first, use insulation to keep the passenger compartment insulated from heat and sounds second, and worry about panel-sourced vibration a distant last. The kinds of amounts of noise that Dynamat is effective in solving in production cars is a very small part of the noise and heat equation for our cars.
