I've found there's kind of a science to XP sharing. For starters unless you have vitality in it when playing the leveling toon, Keep it OFF, Having it at 0 vitality and enabled drastically kills your lower level toons XP. So I would suggest, Enable it. Park the toon somewhere or even offline while (raiding, grinding other groups ect..) to get the vitality filled. Once filled, Play that toon with your group for the boosted XP. When the vitality empties, Turn sharing OFF on the character your leveling while playing. Rinse and repeat.
When I say it drastically kills XP, I'm guessing its around 40 - 50 % drop when enabled and empty of vitality. At least that has been my experience with it.
It was my understanding Fellowship XP sharing was developed for your friends that cant play as much as you did, to allow them some sort of boosted leveling when not around so they would not fall behind you. I'm not sure it was developed with power leveling a online toon all the time in mind. Hince the issues it has in that scenario.