What will happen is the level 51 quests will start pumping out gear slightly below the last tier raid, by the time you get 55 you will start seeing gear better then the last tier raid and when you get to 60 you will have totally replaced your raid set. When an expansion comes out they want to close that ilvl gap between players and do a gear reset, they aren't going to give you ilvl 51 gear from 51 quests that wouldn't make sense. I think some people are still hung up on the idea that character level = ilvl, this isn't true.