Okay, you guys are correct that DH has higher raw healing. I see where I went wrong with trying to calculate it. The healing buff it gives is good too, I just didn't know how to calculate it in.
This is all theoretical though, since in practice all the throughput CDs will always overheal. Every fight I looked at had overhealing. Looking at Brightmist's guild's Thok kill, Brightgleam (not sure if that would be you) used two DHs for 10.5 million healing total (the DH healing buff would obviously already be calculated into this), with 21% overheal. Add another 4.2 million EoL at 40% mastery (and this is assuming none of it overheals, which will lower the overall value). That's less than the proposed 18 million. Most fights have a lot more than 21% overhealing from DH.
I don't really think it's really fair to call the 10% healing half a Barrier considering how much that 10% will be susceptible to overheal, especially once it starts stacking with itself. Is it good ultility to have for your raid? Sure.
So while yes, DH is stronger in theory, I still think Barrier is the better choice, unless you run with all Disc Priests, Holy Paladins, and Monks. We are simply looking at two different things. You guys the healing potential, me the realistic healing it provides. It's value will likely go up more in heroics with less overhealing, but so will Barrier with more incoming damage.
EoL works off raw healing. So the 4.2m EoL you are calculating assumes already that EoL overheals an equal amount with DH itself. Realistically EoL has a slightly higher overheal on thokk, so you are looking at 3.5-4m healing. Still that is way more than barrier in a real fight. (barrier won't break 5.5m on thokk).