We've datamined how much mobs were buffed and nerfed at every level from 68 to 79, which now also includes Item Level Scaling!
Best Hero Talents & Talent Builds for Leveling
Below, you'll see a table of how much health and damage the mobs gained (or lost) at each level at a baseline (see below for addition modifiers).
*This only applies to players with the War Within Expansion
Blizzard has now added Item Level scaling to leveling in War Within.
Now, some significant Item Level scaling nerfs have been added from Item Level 342 to 571. Item level scaling above 571 seems to remain the same.
This means that if you are item level 342, mobs will have a 54% nerf to health and damage.
Note that these are NOT thresholds, but a linear function so if you're 366, it should be ~43% Nerf.