So very informal test running battlements and little bit of keeps in act 3. I selected two runs in inferno mp 1 and ran one in hell mp10. The monsters definately died easier in hell but the rough experience difference was not as big as I expected meaning I got about 14% exp from two inferno runs and about 6% exp from the hell run. I'm pl 28.
So I guess my question is what do people think about using mp10 in hell to pl faster than inferno or at least more efficiently? My next thought is to try hell act 4 because although fewer monsters their levels are more similar to 60 and the exp penalty for too low of a level should be less should be less.
Maybe in the case of paragon leveling it's more efficient to go to hell.
Although I have the feeling most will stay in inferno, because of the MP changes the drop chances are increased too much to ignore it.
I don't know the numbers, but I personally think that Act 3 Inferno MP0 is still going to be the fastest PL level area. Obviously you move up MP levels as your run becomes more about move time than kills, but I think the monster exp is still better in act 3 inferno than anywhere else.
If you want to compare you need to count exp/hour, not exp/run. exp/run is meaningless. Try act 3 mp0 for comparison, as for most gear levels that is the most efficient place in terms of exp/hour (you can have higher MP, though, as long as you still 1-shot all normal monsters).