I believe MP4 and MP5 are the most efficient Monster Power levels to loot items.
Ok. So we are trying to find which MP level is the most efficient in giving loot.
You do the same amount of damage in all MP levels.
You can handle the incoming damage in all MP levels.
Length of runs are directly correlated to Monster Health
The first assumption is for a controlled experiment. Nothing odd there.
The second assumption assumes the additional damage doesn't kill you. As monster dmg can increase up to 371% of base dmg, this assumption is not really safe. Death reduces the efficiency of loot (as it increases the time taken).
The third assumption assumes that the only consideration with more monster hp is more time taken. This is reasonable only if assumption 2 is. For bursty builds that do not expect to engage enemy units for long, having them live long enough to counterattack is a big difference. The resource constraints for each class also make it such that classes that can run out of resource in extended combat will experience more problems than other classes. Furthermore this assumes that the base damage is low enough that there is a significant increase in time. If a player can attain overkill in most of the monsters this assumption would be violated.
When you take the increase in health, and divide it by the bonuses you get with each MP, MP4 and MP5 get roughly the best bonuses. After that, the health of the monsters scales up far more than the additional bonuses.
Proposal: MP4/5 give the highest additional bonuses per additional monster health.
This does seem so yes.
I'd bet with a large sample size and the best characters you'd see the best returns on MP4 or MP5.
The limitations of this theory are as follows.
1. Does not take into account resource constraints (assumption 3)
2. Assumes that incoming damage is trival (assumption 2)
3. Assumes that the positive correlation in mp level and time for run is significant (assumption 3)
So when would this theory work? For a player who has no significant resource constraints, is largely immune to enemy damage or resilient enough that the chance death is low enough to be ignored and whose damage is not high enough to overkill enemy mobs across different mp levels.
Even with enough gear and player skill to avoid the first 2, I don't think assumption 3 is reasonable at all.