In the loot.xml, this is the line that seems to affect POI bonuses
<loot_settings poi_tier_mod="0.05,0.1,0.15,0.2,0.25" poi_tier_bonus="3,6,9,12,15" />
For biomes, it is in the biomes.xml file
<biome name="wasteland" topsoil_block="terrDestroyedStone" biomemapcolor="#ffa800" lootstage_modifier = "1.5" lootstage_bonus="30">
The modifier is the percentage while the bonus is a flat addition