I'm going by the base game mechanics here, not purely by the GECK.
DT reduces all damage that falls under the threshold.
So tripling DT would make DT much more of an all-or-nothing affair.
Consider the following example:
An enemy with DT-10. A weapon with Dam/shot 9 and a weapon with dam/shot 15.
If you triple DT and DAM both then the weapon with Dam/shot 9 will deal 27 damage. Against an enemy with DT 10, tripled that's 30: The result is the shots still do minimum non-penetrative damage.
The Dam/shot 15 weapon will deal 45 damage, compared to DT 30, 15 gets through rather than 5.
Lets say the mob has 100 HP, for round numbers.
Before modding the Dam the Dam-9 weapon does minimal damage, lets call it 1%. The penetrative dam-15 weapon does 5 damage after DR, or 5% damage.
After modding the Dam-27 weapon would cause still minimal damage, call it 1.5%. The penetrative dam-45 weapon would cause 15 damage, or 15% of HP.
So as you can see the sub-penetration weapon increases damage 50%, the penetrative weapon increases damage 300%.
The resulting mod would mean if you can't punch DR your weapon is slightly better than vanilla. If you can punch through DR your weapon is exponentially better.