On MP1 you get 10% bonus XP, but mobs have 50% more health. The bonus to XP is added to what you already have, which makes it less of an advantage than you would think.
Looking at my own gear the difference is as follows:
Start %XP: 96 (35 from Hellfire, 7 from followers Hellfire, 23 from Leoric, 31 from ruby in helm)
End %XP: 171 (96 from gear, 75 from full nephalem valor)
Start %XP: 106 (96 from gear, 10 from MP)
End %XP: 181 (96 from gear, 10 from MP, 75 from full nephalem valor)
So during most of my run (in particular fields of slaughter and keeps lvl 2) I get only 3.7% (2.81/2.71) more XP when at full neph valor (compared to MP0), but mobs have +50% HP. Even if you have only modest XP from gear (perfect star ruby in helm and a Hellfire, for a total of 60, plus full neph) MP1 will give you only 4.3% more XP than MP0. Seems like a bad deal, and testing shows this to be the case also. For exact results you must test this yourself, for example doing 5 full runs on MP0 and 5 full runs on MP1 (etc). Calculate XP gained per hour.
In short: to maximize your XP/hour, play on MP0 unless you have realy good gear. A big advantage to doing MP0 is that you can wear weaker gear (less str/vit/resist) and add MF/GF/pickup to also gain a substantial economic boost. I'm gaining something like 55 million XP, 600k gold and 2-3 legendaries per hour, doing Alkaizer runs on MP0 with close to maxed out MF and GF. My gear is pretty cheap (besides the Leoric ring which is 20+ million on AH) so anybody can do this with a barb.
Very interesting information from you... I think i'm goign to drop to mp0 as it takes me about 1-2 mins to kill some elite mobs