Hi,
The setting for grenade damage appears to do nothing at the moment.
On default settings, a frag grenade deals 400 damage to a sandstone wall.
On 0.1 for both, the frag grenade also deals 400 damage to a sandstone wall.
On 0 for both, the frag grenade also deals 400 damage to a sandstone wall.

Tested on Rimworld 1.6.4566 rev606 with all DLC enabled.
Mods enabled during test (in order):
Harmony
Core
Royalty
Ideology
Biotech
Anomaly
Odyssey
XML Extensions
Walls are solid

Using RimPy for mod manager if that matters.
Hope this helps, thanks for the great mod.