Adding more HP's to undoing the changes to the attacks the mobs use, is not a significant retuning, after all they actually went in and changed them to make them easier, so with your reasoning, WOTLK could be out now with the time they saved?

They went too far with the nerfing, plain and simple.

I think you are vastly oversimplifying the entire process of balancing the raids. It's not a case of just simply adding more HPs. I think they already said that the raids in their current form would have been nigh impossible with the changes made to potting, mana regen, etc. So instead of going back and trying to determine how exactly how each raid encounter has changed and how much HPs to take off, and if currrent talents negated the bosses abilities, they just cut all HP by a fairly heavy amount so that people could do the encounters without too much trouble. Without having to waste time re-balancing what will soon be redundant content the other alternative would have been to release nothing until the expansion. This would have been bad for them as they would not get the chance to see the how core content behaved in a live environment, and bad for use because we would not have had the chance to play with all this new stuff for free.