Once someone outside the boosted party damages tagged mob that is grey for him, the character in the boosted party gains just a small fraction of XP from the kill. This was already 1.12 mechanic IIRC. It would need some tests to find out the exact figures. I have seen yesterday a lvl 60 damaging mob tagged by level 10 and the level 10 gained only around 10% of the full experience.

However, when the killed mob is at least green for the high level outsider, then the boosted one gains full XP from the kill. The last green mobs for 60s are probably level 45, which would be effectively tagged by lvl 43. This would mean, a 60 can boosts characters from lvl 43 up for the full XP.

To cover the rest of levels one would need another outside boosters at following levels to avoid this penalty:

  • 43: to boost from 29 to 43 by killing mobs from 31 to 43
  • 29: to boost from 17 to 29 by killing mobs from 19 to 29
  • 17: to boost from 7 to 17 by killing mobs from 9 to 17
  • 7: to boost from 1 to 7 by killing mobs from 3 to 7. Or simply level to 7 traditional way.