Yes, I agree with it to a degree, the biggest difference would be from the ducting, though. It has a lot of active air(huge convection potential) flow, and much, much more surface area than your hood, besides having a huge uninsulated bottom, where the light and heat will radiant out into the room.
The heat generated by HID is mostly radiant, as in light waves, or rays.
For your ducting, they make foam insulation wrap(you could use bubble wrap, air is excellent at insulating). Could probably use clothe, there are lots of insulators you could use. Put whatever one you like over the original ducting, and then put a layer of foil around that.
Aluminum doesn't give off heat easily(it is not like a heater), it has an IR emissivity of around 3%. Which means it gives off 3% of it's heat as IR radiance. Then again, this is usually why the outside of the reflectors are typically painted white, because white paint has an IR emissivity over 90%, typically. The aluminum conducts heat to the paint directly(highly efficient), and the paint is capable of dissipating large amounts of heat.
Most metals have a fairly low thermal emissivity. This is why it requires you to actually touch metal to know whether or not it is hot(even if you put your hand very close to metal that could burn you, you're unlikely to feel much radiance, until you do touch it, and conductive heat transfer occurs, which is the most efficient form of transference, and you're pretty much instantly burned). Unlike, for instance, human flesh, which has a high thermal emissivity, and you can simply 'feel' the heat of yourself or another person when you're within an couple cm or so from each other. You are feeling thermal radiance.
3% is not a shit ton of heat. Insulating the ducting would be quite effective at trapping it, which is why it's already aluminum. The reflector hood? Not so much... and probably dangerous to a degree. How much does the temp go up inside the reflector hood? That's what I want to know.
Double layer insulation is about 99.9% effective, instead of 97%. That'll be your difference, about 3%.