There are many reviews on this topic on youtube, where they compare liquid metal to different thermal paste. In general the answer is that a very good thermal paste will give you a similar (slightly worse) result as the liquid metal. The thermal past will however be way more easy to use and not short your circuit or do other bad stuff. Liquid metal may be an option if you want to build just one or two of the miners for yourself, but it would never be an option for a larger scale production.
See videos like this:
https://www.youtube.com/watch?v=5a9VcG4j8-kThe issue with paste is we can't guarantee it's completely insulating the heatsink from the ASIC.
What about those old school mica heatsink pads? How's their thermal conductivity? I googled around and couldn't find any numbers.