http://www.wowarmory.com/character-sheet.xml?r=Aggramar&n=Sunquan
So that's about 325 MP5 while casting if you have AB, DS, GotW, buff food and Wisdom on. That's about as far as I can get.
Now, change him to a deep Disc spec with Enlightenment. You might be able to squeeze another 16-17 MP5 out of it, but that's about it. And it's grossly inefficient for a main healer.
Still, 325 MP5 is bangin'.
Edit: Also, compare to another priest who has cleared BT:
http://www.wowarmory.com/character-sheet.xml?r=Aggramar&n=Syome
296 base, about 310 with Int and DS, 355 with Wisdom. Getting closer.
Edited, Apr 2nd 2008 5:01pm by TheEngine