I've measured my exp/hour, and find MP0-MP2 give roughly the same rate. I also noticed more legendary drops at MP2, but that might just be down to RNG/subjective experience. If exp and legendaries are most effieciently farmed at different MP levels, I'd personally choose a level somewhere between them. Paragon levelling is less fun without decent drops to keep me going.
Are there advocates for MP0 wizard levelling, or do most people play at the highest MP level they can, without losing time (i.e. still able to insta-kill most trash mobs)? Is there a definitive answer to the question of efficiency for exp and legendaries, and has anyone done the maths (or long-term measurement) of legendary drop efficiency at different MP levels?
I made a spreadsheet to calculate equivalent run times for paragon levelling and legendary drops at each MP, based on your time for MP0, 1, 2, 3 or 4. This should give you an idea of targets for maximum efficiency.
http://bit.ly/MP_efficiency (download to edit)
Fill in your bonus exp/MF info, and run time for a given MP, to calculate the equivalent run times for the other MP levels. The legendary drop rate scales much more at higher MP, due to bonus items.
E.g. for a given 18-minute run at MP2 (total exp 276%, MF 288% at 5NV), an equivalent run at MP3 would be 0.7 mins longer for exp, or 2.9 mins longer for legendary drops (assumes ratio of gear drops from elite:trash mobs is 2:1).
Calculations for legendary drop rates are borrowed from Shelendil's spreadsheet: