The original grey Sil-Pads are only good for low power applications. As you go up the product line, the thermal performance becomes much better. Unfortunately, the price goes up accordingly. The better (expensive) pads can be used in fairly high power applications. You never apply additional compound to these pads and I think the manufacturers are usually clear on that. They also should be used once only, but I admit to cheating a bit there. Trying to compare insulators is fraught with difficulty. You'll find that even within a single manufacturer's product line they sometimes change units or test conditions. Or the thickness is different and they don't compensate for that. Mica and grease is great if 1) you can stand the mess or not make one, 2) the mica is very thin. Some mica is remarkably thick and mica is not a good thermal conductor. Sometimes you can cleave them thinner. Actually, not much is a good thermal conductor, other than metal. Grease isn't very good either; all our thermal interfaces rely on being very thin. Hard anodized aluminum pads and grease are excellent, but you have to be religious about burrs, sharp edges and voltage ratings. Remember that not all audio applications need maximum thermal performance. If the devices don't get very warm under the worst case conditions, the pads are plenty good enough.
Henkel bought Bergquist so the data sheets are a bit harder to find now but look for the Q-Pad II as an example of a high conductivity pad. Digikey will have the datasheets and selection guide.
