For the HLOD Settings:
When you have Simplify Mesh on, you can control how much it will decimate/reduce polys (Screen Size)
So for example, my HLOD1 has a Screen Size of 500, and my HLOD2 has a Screen Size of 150,
So in the end my HLOD0 have: 3320 polys, HLOD1: 2658polys HLOD2:1318 polys
Hmm it really depends. It will improve FPS, since HLOD is reducing drawcalls, both for static meshes and materials