No idea sorry.
I've been giving a try at the insane difficulty with an oozemancer (looks like insane is starting to be balanced for such class, but it's still strong even at that level as out of a few rares so far no real problem ) , and it seems a bit harder for him than in nightmare (enemy with more health, hit very harder and they have a couple more levels), he cleared all tier1 and just cleared old forest.
He died 2 times in the bandit tunnel, stupidly both times.
The bosses seems harder as they got some skills they didn't have before, Wrathroot was phase dooring without cooldown (similar to that overcheating Linaniil) all over the screen at every turns (i would hate playing a melee class against that kind of annoyance). And even Eidolon had ... a golem
The leveling is faster than in normal but not sure if it's faster than in nightmare, but for the leveling i got i didn't found anything better than i would in normal difficulty (i think i found better stuff at that point in Nightmare), not sure if it was me being unlucky or if it's setup like that.
But it looks i'm reaching my treshold of "replaying the same thing again and again", as i'm not feeling playing more of it, some dungeons could sure lose some levels considering how repetitive some of them are.