High MP Regen doesn't have to be too high but high enough that your skill will cast when you run out of mana.
As for Energy mythic , you gain DMG % from lost Energy so if you tried to use up 50% Energy, you gain 50% more DMG. If you used almost all your Energy aka 90%, you gain 90% DMG. The damage is based off missing % Energy out of 100%.
Primary skills use 10% energy, special skills use 30% energy. The energy does recover every second though but a cast will give you extra DMG , though having the casts being extra costly make you deal overall more DMG.
Example: Barbarian + Energy. I noticed that you half the mana for energy so suddenly special skill ends up using almost all your Energy instead of just 30% of it with 50% barbarian.
With push the limit 100% or 50%, your cost of skills in energy uses more Energy but you deal more DMG as well.
Energy used to be based on full Energy % but that was changed to missing energy % which is fine .
Fury increases special skills DMG by how much Fury % you have so full fury would buff the process by 100% DMG or special skill cast as well. Primay skills don't gain DMG but they do fill up the bar constantly so you wouldn't worry about running out on high APS builds.