As you know, lots of expensive legendaries are pre-level 60, for example The Favor of Time, Andariel's visage and lots of other ridiculously useful items.
Basically we know that act1 inferno mobs are mlvl=61; andariel's visage ilvl is 58 (or 59, can't remember). The question is, do we have a higher drop chance in act4 hell or in any inferno act? Can we assume that ilvl61 legendaries have a higher chance to drop (with high mf) in act1 inferno than ilvl55-59 legendaries?
Well, at least for wizards it really stops at Siegebreaker. Going through phasebeasts/lashers on inferno isn't really possible as wizard unless you're gearing BOTH for damage/kite (attackspeed/movespeed) and very high resistances/armor, which would be ridiculously expensive ;\
it might be viable to corpserun though, however act3 farm build is very different from azmodan solo build (for wizards)
anyway, it all comes down to the fact that you can easily farm act4 hell in full mf gear without any troubles