boosting XP rate lower?

[Ualaa]

Let's say you get 90 exp, without the level 70.
And you get 22 exp with the level 70.

90 divided by 22 is ~4.
If you can kill 4x faster with the 70 in the party, then its a good move, provided you don't run out of mobs.

If you're in a densely packed area, such as an instance...
Or if the mobs are 4+ levels higher then the group (so they'd be impossible in groups without the 70)...
Then the higher level is definitely worth while.

[nerfshot]

just a quick time for any new people boosting. When you turn in guests be sure to DROP your high level group member this way you will not incur the xp penalty from having him in the group and you will get true 300% bonus.