So, I've been playing with the Demon Invasions on the Live servers. At first I intended to just get Achievements on my capped characters, but then I read about (and then watched) folks leveling sub-100 alts via this method. So I watched a few videos, and had to give it a go.
Ho. Lee. Shit.
This is insane, especially after the most recent hotfixes, and it shows in the crazy level gains. My long-languishing Monk? Ten levels over two events. My three Mage alts? 9 or 10 levels from ONE event, reflecting my better understanding of how the mechanics work. A LOL Gnome Hunter I rolled just to see how nuts this is? 10 levels with ease in one go. My Dwarf Warrior alt? 10 levels in one go.
I have the advantage of full Heirlooms, but no other XP boosts. I want to see how fast this can go, with three points for me to stop leveling: 60, 90, 100. At 60, that alt qualifies for the Veteran Bonus should I boost to 100. At 90, I can speed-run that alt through Draenor; the means and the route are well-known now. At 100, that alt can go direct to the Broken Isles. If I want to set up a Demon Hunter on another server, getting an alt to 70 is the goal, but I'm good as I am so that is not a consideration.
The key is to tag every mob, and then not die; the latter matters be XP-for-mobs isn't a thing while dead and you get a lot of it. Mobs in these events scale, so tagging works great and it's open tagging so slap all the things. Mad gains to be had, and if you go all out on the XP boosts you can speed-level in hours.
Also, save the chests until that alt dings 100; gear gotten from them also scales to your level when opened, like Timewalking gear does.