There are at least two ways that cryo improves CDs. First, it makes the CD stiffer and less prone to vibration by improving the physical characteristics of the metal layer and the clear layer. Second, it improves the physical characteristics of the clear layer, making the laser reading more accurate.
Heat treatment is also tempering, like cryo, and actually improves the physical characteristics of metals. This is why tools and swords and such are heat tempered - for better durability and strength.