Hum... I never played XI, I have no idea how it works there BUT, I played City of Heroes which features level sync as well (both ways, both lowering the high-level's level, called examplaring, and raising the low-level's level, called sidekicking).

It does't intereferes in any way with levelling? Because both the level-synched and the normal-levelled persons continue to earn XP in the same rate as if they were fighting level-appropriate foes, so even if my L10 hero is sidekicked to level 50 I still earn experience at the same rate I would at level 10, so there is no such thing as sped-up levelling. All it means is that Ican hang hour with my overlevelled buddies.

How DID it work in XI so that you can explain it to a poor old me?