Since the patch, running with my Monk has been pretty productive when it comes to MF. I do a Cydaea to Diablo route...
-Start the "Second Heart" branch of the "Heart of Sin" quest...
-Keep Depths for your stacks
-Stonefort for an Act 3 Key
-Tower of the Damned 1 (optional, if you have time, it's a short circle but worth a few elite packs)
-Tower of the Cursed 1 and 2
-Core of Arreat
-Head to Act 4, Kill Iskatu
-Northwest, Kill Rakanoth
-Find the Hell Rifts (good for a handful of elite packs when you get a system down for finding the rifts)
-Silver Spire 1 (easily a good couple of elite packs)
-Silver Spire 2 (another good couple)
Obviously, it's not a speed-run style "run-rinse-repeat-run-rinse-repeat" kinda thing, but if you have the time, it's just a straight path to the end, and Act 4 is really underestimated as far as elite pack quantity and drop quality. This past week, I did it only a few times through with my Monk on MP2 and came away with about 4 legendaries, 2 set items, a Marquise Ruby plan, an Intelligence Amulet plan, two Archon Shoulder plans, a few other gem plans, and a ton of lower level crafting plans. For real, Act 4 may have more space between elite packs, but if you get a good system of finding the Hell Rifts and make it to the Silver Spire quickly, the Spire floors are packed with elites.
I actually can't wait to do this route again, to be honest, :-)
I like the Cardinal's run myself. I wasnt really looking for "speed" runs. I picked up part of the Tal Rasha's set, IK gloves and some cheesy level 58 unique Wizards hat today doing some various runs. Thanks again.
The runs for xp efficiency are going to be the same as the runs for efficient mf for legendaries. MF chances are directly proportional to mobs killed/time which is the same thing xp is looking for. 75% bonus MF notwithstanding, I think that killing the dense groups of mobs lead to far better chances to find legendaries than full clears. If you think about it, a 300% MF base plus 100% for mp4 and your base 100% gives you 500% of the legendary proc for each mob anyway, so a 75% bonus increase is only a 15% increase in total chance to find a legendary. I guarantee that the mob density in the big 3 or big 6 areas will more than make up the 15% difference you get from doing a full run after 5 stacks of bonus mf. If you can do a full clear of act 3 in 20 mins, I can do two big 3 runs in that same time and will kill more than 15% more mobs than you.