If only I could find the person who commented on Shrapnel Thresholds being dependant on DPP.
The higher the DPP means you would loot lower the amount of shrapnel, as the examples below.
2.9 = 60% Shrapnel / 3.0 = 50% Shrapnel
3.1 = 40% Shrapnel / 3.2 = 30% Shrapnel
3.3 = 20% Shrapnel / 3.4 = 10% Shrapnel
Based on your main question. "I want to test either a
low decaying gun like i2870 / mod merc or a high-efficiency wep like EP-38 Night special Aug."
From what you laid out in your post, I understand you have tested several 1.0 weps with similar stats and now want to move to low decaying 1.0 weapons, moving on to later test a few 2.0 weps.
This is all to find out what setups would produce the highest amount of shrapnel at the highest efficiency level? If I am correct, then great, following so far.
Suppose you want to loot the maximum amount of shrapnel. In that case, I will use a low DPP, high-efficiency setup with an overamped amplifier and use the highest decay attachments possible, i.e. Scope Longreach 4, Sights Bullseye 8's. That's how I would do it and don't bother using any Extender.
If you want to profit in this game, I will go with a gun with a meagre decay rate vs ammo consumption; the closer to imk2, the better and keep shooting without stopping.
Please remember that DPP determines loot composition. Looter & Efficiency determines the amount of TT you'll get back, which for most Loot 2.0 or 2.1 high Eff weps will only give you an added benefit/return of say between 1-2% TT extra returns over a Loot 1.0 with 64.29% efficiency.
If you genuinely want to get higher TT returns, the best course of action would be to use a Loot 1.0 setup, highest DPP (with buffs ofc), not overamped, maxed, with Looter profession above Lv36 as a starting point.