Reflective Mylar is just a very thin layer of highly specular(mirror-like) aluminum on plastic(PET, polyester) fiber. Specular reflection is actually reflectivity according to the law of reflectivity where the angle of incidence is equal to the angle of reflectance.
Flat white paint has negligible specular reflectivity. It is a highly diffuse surface which scatters light omnidirectionally off the surface. This means less target lux(lumens per area), as it's the same energy, but spread over much more area.
Aluminized Mylar has very little diffuse reflection, less than 25%, typically. Conventional aluminum foil is 'balanced' @ around 50% diffuse. Both are around 90% specular(visible light) when polished, as both are aluminum.
Diffuse and specular reflectivity are not proportional to one another by any means. Many materials can be highly efficient at producing both types, like aluminum. Also enamels and powder coats(most white reflectors).
Not many people seem to mention this, but reflective Mylar is combustible and will burn rapidly when/if ignited. Pure(or nearly) aluminum(like most foils) do not burn. And Mylar is certainly more likely to create 'hot spots'. Just as mirrors can cause hot spots.
Mylar isn't intended to be around high heat sources(over 150F), keep that in mind. The bulb temp of a HID lamp could easily melt or possibly ignite it.
It's made of mylar and works great. These things are meant for survival so they don't fuck around when it comes to reflectivity. I have to wear sunglasses when I look into my closet now 
