Okay, I see what you mean now. But with the level of coordination you're talking about here, it might just be simpler to have the mage start casting their Pyroblast (I've been wracking my brain, but I honestly can't think of any other long cast, high damage spells), and then during the last 3 or 4 seconds, start the pull - if aggro isn't going to be an issue on the target within that time frame, then you can save the RD if something else goes wrong within the first few seconds of a pull (which is where, at least in my experience in five mans/heroics, is where the majority of pulls will go wrong), and still get that high damage spell off to frontload dps.
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Yinramu - Hyjal