Just looked at my logs from last night, and it looks like I'm not receiving the full, or necessarily any benefit from Demonic Aegis, where on Omnotron my FA ticked for 153K, our Destro Warlock who isn't specced into it received 121k from his FA, both including overheals; he did overall more damage but largely that was through Bane of Havoc which I'm not sure should be proccing heals. Also, on Argaloth, I did 60k healing vs his 50k. Either way, although I received more healing it doesn't come close to 50%, let alone 100% more healing.
Judging by my logs from yesterday it appears to be working correctly. Keep in mind that it only counts your own single target damage: Your demon's damage doesn't matter, and as Demo a larger part of your damage comes from your demon. Just taking data from the raiding I did yesterday:
Destruction (No Demonic Aegis), Argaloth(link)
Single Target Damage: 2842448
Expected Fel Armor Healing: 85273 (3%)
Actual Fel Armor Healing: 82667 (34307 effective, 48360 overhealing) (2.9%)
Demonology (With Demonic Aegis), Maloriak(link)
Single Target Damage: 3477352
Expected Fel Armor Healing: 208641 (6%)
Actual Fel Armor Healing: 207243 (113362 effective, 93881 overhealing) (5.9%)
Both seem fairly close; slightly lower than expected in both cases, but I might be counting some extra damage sources that I shouldn't. It's a shame I don't have a better fight than Maloriak to look at for Demo unfortunately; I only really spec it for a few specific fights.