other than collection quests, leveling 5x at a time can go faster than leveling 5 sequentially. so yeah, the per kill exp goes down, you kill that much faster, and get exp on 5 toons. I have multiple teams so I try to level each one differently to avoid the monotony. I leveled 3 separate teams to 60 3 different ways, well 2 were both questing, but I did mostly different quests. The third team was boosted.

What I would do, is quest as normal, but drop any collection quest (unless you really care for whatever that quest leads to) and make up the lost exp with a couple instance runs. I plan on alternating teams with rested exp, so one of em is always full rested.