a good quality mirror can reflect 96 to 98%,.. a high quality mirror can do 99.9%. I've never looked into a household mirror and seen a reflection that was 80% of what was shining into it. To my eyes it's identical. The reflection is the light given(reflected) off by an object bouncing off the mirror. You would notice if the reflection in a mirror was 80% dimmer.
A mirror is nothing more than a piece of glass with a reflective material attached to the back. It's virtually impossible to get a "hot spot" with a flat mirror. A "hot spot" is when the light is concentrated on a small spot,. like a concave mirror would do. Foil can accidently have these types of shapes made into it if it's not flat. It's similar to the effect of using a magnifying glass in the sun.. It's also the same principle large telescopes use,.. concave mirrors to concentrate the light/reflection to a smaller spot. If you use flat mirrors the only hot spot you could create is in the corner where two mirrors meet, and that's not concave nor will it concentrate the light enough to burn anything,.. In other words, if you're corners are burning your plants, the rest of your grow is probably going to cook anyway.
The quality of the glass has an effect on the percent reflection.
It may add to the heat of the grow, by reflecting infrared light or heat energy back into the grow. Light bulbs give off heat energy as well as light energy. This is one reason why you get more lumens per watt when you go above 400W with HPS,.. they get more efficient at convert more electricity to light. I don't see a white wall preventing the heat, because it would both reflect it also (just not as efficiently) and heat up slightly-warming the grow,. The same principle used in vacuum thermos bottles. Your hot liquid gives off radiant heat (like a light source does) and it get's reflected back into the liquid by the mirror finish in the thermos.
White walls reflect enough light,.. mirrors, mylar reflect more. Not many people have the problem of too much light, and if they do, there's no need for reflective material at all.