It just lowers dpp proportionally to how much you overamp, I.e. putting a mayhem delta on a tt gun, will turn that gun into a grindhouse (but at even lower dps).
No it's not lost ped, it's just dec dpp and increase eff(depending on the peashooter you're using)
Easily tested and confirmed by multiple people.
It's why it's okay to use higher mayhem amps(L) if their mu is the same as long as the ammo use is same. As each of them have the same amount of uses for their weapon type.
It's also why it's funny to see beta/gamma amps cheaper than the lower amps sometimes. Happens often for weapon types that don't have nice dmg intervals for certain Mayhem L amps.
But moral of the story, use whatever Mayhem L amp to overamp your weapon you want iwth the lowest MU, although pretty sure all are basically 0 mu at this point. It's literally free dps/eff/heals. With them being so close to tt. Should ALWAYS be used on loot 1.0 weapons. Loot 2.0 depending on the weapon/what UL amp you have, but at this point I'd prob always Use L over an UL for amps with prices(somehow) still droppping.