"No disrespect here rockm, but I'm wondering why or under what circumstances will the Napa stuff turn into mush quickly?"
Non taken. There has been considerable regional variation in Turface and other fired clay products where one person has a batch that turns to mush after a winter or two. Adding to my caution is that the Napa product is aimed at absorbing liquid and then discarded. It's not made to stand up to weather, in other words. There is, IMO, a lot of room to think hard about using it in bonsai soil.
The biggest question I have about it is using it in temperate climates. The freeze/thaw cycles in Pa.-- and most of the eastern U.S.--is a big issue for soil ingredients. Freezing is a destructive process for porous materials that may stay wet through the winter. The water inside the particles freezes and expands, which pulverizes them pretty quickly. I've seen turface break down in only a year here. I can't find any listing of how Stay Dry is made. I don't know if it's fired in a kiln at all...High firing increases durability.