Here is an update to this thread. I've been researching Silver Mica caps for a few months now. The more I looked at them, the more intrigued I became.
If you need a HF bypass, as in RF, it is hard to beat Silver Mica. It beats COG/NPO ceramics in every parameter, except cost. It has very low loss, very low ESR, and very low ESL. For audio work where small caps are needed, they also do very well. They are not microphonic, they are sealed environmentally, and have low ESR & ESL characteristics. The plates are silver, hence the name.
Unfortunately, I was not able to find any information or tests indicating whether polystyrene or silver mica is better for audio work where RF is not a question. However, for RF bypass, such as phono inputs or amplifier inputs, they are clearly the best choice available. Polystyrene and Polypropylene do have higher ESR & ESL numbers than Silver Mica, so they are less suited for RF bypass applications on inputs.
The only drawback is cost, these puppies are pricey! Expect to pay 10x to 20x more than an equivalent COG/NPO ceramic cap.