The ingame explanation of the different difficulty levels states that the Torment difficulties increase the drop rate of legendaries. But they don't state exactly how large this increase is.
I've farmed a bit on Normal difficulty and found some nice stuff, but I'm also roughly able to farm at Torment I difficulty. I'm wondering if the large decrease in speed compared to Normal is worth it for me, but to judge that I need to know a bit better how large the increase in legendary drop rate is.
Best Answer
From the diablo forums, the legendaries drop rate is not disclosed as of now.
Reading the whole thread of the forums, the difference of drop rate seems low enough to not warrant that a fast run would get you less legendaries than a slower at a higher difficulties. If you make a run at Normal a lot faster than at Torment 1, stay at normal. You might wanna try Master or Expert since the differences shouldn't be drastic if you can run T1.
If it only takes you 10-15 more mins at T1, go with T1. Only with tries out will you know which setting will net you more legendaries. For my part, I'll be lowering to T1 since T2 is a tad too slow for my taste even if I got 4 legendaries in about 4-5hrs of play.