You got me curious here.
So i went to sisyphus with my spirit + dante (92 dmg) and (just) 40 ped ammo.
The first 35 ped of ammo I shot using a 4 dmg finisher, getting next to no overkill.
2 spirit shots usually leaves just a sliver of health on the defenders.
Did not loot a single item.
Then with my last few shots I didnt use the finisher and just shot 3 times, so getting around 50~90 overkill.
First mob I heavily overkilled gave a 36 ped global (including full TT LLC1).
Then nearly each kill with huge overkill gave me an item.
With the last 40 ped (without finisher) I looted 6 B101's, 6 Azuro's, 6 lanceheads and 2 EP2's.
So, first 40 ped without overkill using a finisher, not a single item.
The last 40 ped with overkill without finisher, 20 items.
Now I know it's just a small sample size, but this seemed quite extreme.