We now also have more than one Resto spec Druid coming along too (thankfully )
That helps tremendously with healing....with my gear and spec, as long as I don't put all of my fire resist gear, my rejuvs tick for 310 each time. I can, on a crit, heal nearly 5k hp with healing touch. That makes a huge difference in the healing area. We also have more Druids along now and that also helps.
I don't think anyone is saying that Shadow priests can't buff or heal as good as the other spec...we know they can with the right gear. I think it comes down to at times seeing shadow priests dps'ing when they should be healing. It's like Shaman and Druids too...they too have dps specs but are able to heal. Anytime I brought Huato when he was specced elemental or enhance I always stuck to healing when we fought bosses and the Molten Giant/Destroyer combos since they have such a high damage output potential on the tanks. While it's more important to heal on the bosses, lava packs, and giants encounters, everyone should be helping to heal their groups during every trash mob that can heal. It keeps the raid alive and moving through the instance faster, and also helps keep down the repair bills. Plate wearers have it the worst, then it goes down from there based on type of armor, but it still isn't cheap. Zuato's leather is cheaper to repair than Huato's mail, but it still gets costly in a hurry if I die alot. I cringe to think what our tanks have to pay when I can't get a heal off in time.
What it really boils down to in MC, BWL, Onyxia (to a lesser extent), AQ40, and now Naxxramas is that the healing classes have to heal for the raid to be successful. You can still dps in the 5/10/20 mans, or when allowed to by the raid/group leader. We're not saying that you have to respec or not come at all, just please be mindful of what is going on.
Overall we did very well last night. We had Shaz down by 11:30, and we had a few wipes, but we have come very far since we began.