There are essentially three situations that can occur with this. The DR reduction is lower than your base DR, is greater than/equal to your base DR but lower than your toughness DR, or is greater than both. Additionally, some ammo types actually raise the targets DR, and so does the Finesse perk.
If the ammunition has greater penetration than both your toughness and non toughness DR, then unless there's a downside to having negative DR (I don't believe this does anything, but I could be wrong) toughness will have zero effect either way. In this case, Lifegiver would have an effect but since it is not multiplied by your DR at all it would always be an effective HP increase of only 20.
If the ammunition has less penetration than both DR values, then we will still be comparing two DR values, but they would both be lower. For example, if the ammo penetrates 40DR and we had 50 base DR with 55 toughness DR, then we will have 10 and 15. As mentioned earlier, toughness gets worse the less DR you already have. Assuming an average character in this case, we have gone from an increase of 75 effective HP from toughness to an increase of 17.65, less than Lifegiver. This is a very significant change, and makes it unlikely that toughness will have an influence on how many shots it takes the enemy to kill you. The severity does depend entirely on exactly how much penetration your opponent has to bring to the table, and the opposite effect will occur if they use ammo that gives you even more DR. Lifegiver follows a similar trend, wherein the effective HP granted by it is reduced as your DR is reduced. In this case you would go from an increase of 40 to an increase of 33.33. In general, Lifegiver will be less influenced both by increases and decreases of DR.
Finally, if their ammunition negates your armor DR exactly you will have a remaining 5 DR from toughness. Effective health without toughness is equal to your normal health, and having toughness will grant you:
100% / (1-0.05) = 105.2631% of your health as effective health.
Ammo types that reduce DR:
Ammo types that increase DR:
Many other ammo types have no effect either way on DR.
Summary of section
-If their Ammo negates your DR entirely (toughness included) then toughness is useless
-If their Ammo reduces your DR, but you still have some then toughness will lose a significant amount of effective HP. For calculation purposes, you essentially treat your armor as it's DR minus the penetration.
-If their Ammo negates your armor DR, but you still get toughness DR then toughness will have an almost negligible effect of only slightly more than 5%