Only thing i hate about the mica insilators are they are a pain when your dealing with 24 power supply fets and 16 outputs and you have to line up each one and make sure it stays in place while putting the clamps back on.
Yeah I really prefer the kapton tape. Only thing about it is that it is very thin and if there are any small burrs or small pieces of debris on the heatsink or transitors it can poke right through that tape when you clamp the transistors down. The mica is a real pain to work with but It seems like I've seen Perry say they are better at transferring heat than the kapton.
Mica insulators and the Kapton MT film are both very good. Mica varies quite a bit in thickness so it's difficult to say, definitively, which is best at transferring heat.
The mica and Kapton are suitable replacements but it's ABSOLUTELY imperative that you check EVERY transistor to confirm that the clamp is applying sufficient pressure to make it lay absolutely flat against the heatsink. The holes in the boards typically allow the transistor to be soldered in at a very slight angle. Since the leads are very rigid and the clamps relatively weak (for those using spring clamps), there is no guarantee that the clamp will be able to force the transistor to lay flat against the sink. Silicone insulators are somewhat more forgiving.
After clamping the transistors, apply additional pressure to the top of the clamp while watching the gap between the transistor/insulator/heatsink. If you can cause the heatsink compound to be displaced when pressing on the clamp, it was not laying flat against the sink. If the heatsink compound withdraws back under the transistor when you stop applying pressure, it's not laying flat against the heatsink and the clamp isn't applying enough force. This can cause the amp to fail prematurely.