My one experience with it was about 2 inches thick. It seemed to heat fairly well at that but another inch would have worked better. Kaowool won't hold up very long by itself as it becomes more fragile and brittle each time it is heated. There are various things that can be used to coat it making it last much longer but most are fairly expensive. It does work well for a quick, short lived setup really well though.
If you use it, don't compress it, the air spaces work as insulating aids. It also is hazardous to breath as it is ceramic and the dust is nasty to the lungs.