I'm boosting a new team at the moment with an 85, would it be more efficient for me to use a level 80 instead? Not sure what the experience loss would be with an 80 vs an 85?

Also, I was going to boost to level 70 and then level normally from that point forward. Is that a wise move, or is it faster to simply quest or run dungeons 60-70 as a team without the high level?