They may have broken it with all the squishes, but I'm pretty sure that ilvl was originally supposed to be 1% increase per ilvl? Something like that? I may be mistaken now, but there for sure was a way to calculate the stat increase because it was straight percentage... saying if and item was 30 ilvl better it had 30% greater stats. You could literally do that math and it would be the exact result.
So in this instance, epic would be 3% better than rare. That's not an upgrade. That's not even a decent xmas bonus.
edit-
10% would be fine. 10 ilvl? Sure, why not? That's at least more than 1 stat point.... often
none last xpac.
And apparently that's not how it works, either thru squishes or whatever "smoothing" they've done to DF leveling... stats are too inflated, at this level anyway. Looking directly at items, I'd say 20% would be a good sweet spot for the bounce from rare to epic... which would be pretty close to the current 86 vs 112 scenario if 1 ilvl was 1% better.