Someone already wrote... once u choose mp1 all monsters becoe lvl 63 and with that they can drop every possible item. Monster power was first implemented to let you control the game dificulty. At the very beginning the game was almost impossible once you reached act 2 inferno.
Now the main point is that the game dificulty raises exponentialy (life, dmg, etc) with mp level and the itemdrops raises linearly. that means that as long as you can 1 shot-kill non-elite monsters you should keep raising the mp lvl. The faster you kill, the more you killand that will give you more drops. Since item roll is random if you can kill more monsters youll probably get more items.
Yesterday I have played MP6 with friends and it dropped like 3 legendaries in 1 hour... incredible... for me it was a proof that higher mps drops better items... no?