These are just instruction counts, so take with a grain of salt. The step counts will matter the most. The cost of the shadows has move to do with number of Shadow Steps you specify.
Parallax only: 176 instructions
Enable POM (slow auto calculation): 234 instructions
POM with manual texture size: 218 instructions
Parallax + Manual PDO size + Shadows: 344
The shadows make the instruction count go crazy. These shadows will be kind of hacky anyways since you have to manually apply them in the base color which does not look right if there are multiple light sources. Hopefully the where PDO affects the main shadow pass will make it in soon so the shadow option can be removed entirely (or maybe it will still be there but strongly suggested to not use).