Trauma, best in slot for any healer...especially raid healers....
So to answer your question, Trauma.
Wrong. For holy priests, Best in Slot is Heroic Dying Light. The issue with trauma is that the proc rate is significantly lower for holy priests verse druids and shamans. That being the case, the stat boost from Dying Light actually produces more healing done then the proc rate of Trauma for a Holy Priest. The math for it is done on Elitest Jerks, if you care to look it up.
Double-wrong. Well, three times wrong anyway. The guy you quoted is wrong, but he's wrong for the same reasons you are.
Best-in-slot weapon for Holy: Val'anyr.
If you don't have it then best-in-slot is:
Most fights: Archus, Greatstaff of Antonidas
2nd place: Dying Light
Any fight with a damage aura: Trauma
So, if you don't have Val'anyr then Trauma is a BiS weapon on 4/12 fights in ICC. In fact it's enough better that Trauma with no off-hand beats Archus on many fights. So the guy you quoted is wrong because Val'anyr is BiS for all raid healers (and will be at 85 if it isn't nerfed). You're wrong first because you said Dying Light was BiS, while Archus is a direct upgrade and second because Trauma is better than Archus, or Dying Light on several fights.
That's why healers and tanks have options lists instead of pure BiS lists. There are too many variables to be considered to call any one set of gear the best you can get. As far as math on EJ? They'll be making far, far too many assumptions. The haste/spirit gain from Dying Light/Archus will not outweigh the fact that on many fights Trauma will be 10-15% of your overall healing. Val'anyr destroys it on those same fights, but that doesn't change that it is much better than the staffs.