Yes and no... MF doesn't help with the AMOUNT of stuff that drops, but does help improve their quality (which is to say, chance for drops to be legendary instead of potion).Quote from Codyksp
I am pretty sure that the higher the difficulty the higher your chances of a legendary dropping, this is clearly stated from torment 1 to 6 but I assume it goes for the other difficulties as well. Either way there is NO REASON to not be playing on at least torment 1, especially once RoS launches (read the goddamn text). If you can't handle torment 1 you are doing something very very wrong with your character.
So if you are killing 3000 mobs / hour on normal and then 1000 mobs / hour on T1 (or whatever gave 300% MF) then you are gonna get the same amount of legendaries / hour.
Later in RoS, some drops will be exclusive for different torrent levels. As far as I know, anything can drop anywhere now, possibly even level 1 skorns for all I know.