I have with help from others found the following methods to increase/reduce mining efficiency:
1. Change harvesting bonus on tools in items.xml.
<passive_effect name="HarvestCount" operation="perc_add" value="0.4" tier="1,80" tags="oreWoodHarvest"/> <!-- DF Titanium Tool Harvest Bonus -->
<passive_effect name="HarvestCount" operation="base_set" value="0" tags="salvageHarvest"/>
<passive_effect name="HarvestCount" operation="base_set" value="0" tags="cropHarvest"/>
<passive_effect name="HarvestCount" operation="base_set" value="0" tags="wildCropsHarvest"/>
2. Change the density/appearance of chosen ore in a area as pointed out by @
Numberz
In biomes.xml change probability as shown below.
<resource blockname="terrOreIron" prob="0.6500" rwgGenerationType="all"/
3. Reduce how many "veins" is created in an area.
In biomes.xml there is many sub biomes in each biome, to change chance of a sub biome being created change this setting:
<subbiome prob="0.95">
4. Change how much ore one block give.
In blocks.xml each of the ore have settings like this. Just change them to get different amount.
Code:
<block name="terrOreIron">
<drop event="Harvest" name="resourceScrapIron" count="105" tag="oreWoodHarvest"/>
<drop event="Harvest" name="resourceRockSmall" count="14" tag="oreWoodHarvest"/>