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I don't know your exact stats, but I'll assume you're at about 3spirit=1mp5, along with epic gems (purified shadowsong or luminous pyrestone).
By taking the Third Eye, you gain 22 healing and ~3mp5, and lose the meta gem. That's assuming you've got Kings and ImpDS. If you don't, then Third Eye is an even bigger upgrade. Third Eye also gives you a little more regen due to the higher intellect, as well as more health. If you gemmed the red socket with a spinel instead, you gain a small amount of regen (~1) and 33 healing.
The best thing about T4 is the meta socket, since with Insightful that's an extra, say, 6mp5, a little more if you're going crazy with the CoH. In the end, you'd be dropping a small (3-5) amount of regen in exchange for 22-33 extra healing. I'd definitely take that, especially considering how easy mana is to come by.
Here is my current setup:
http://www.wowarmory.com/character-sheet.xml?r=Kargath&n=Fingorfin
I'm CoH spec mainly for raiding purposes. I've got some upgrading to do as far as gems go, but there are a couple pieces i would like to upgrade first.