Well lets assume you level an alternate and you never level sync, you just go up in the dungeons as you level. Since you have done most of the dungeons already you are pretty much repeating the same grind with nothing to gain besides EXP. I think it would be cool to be able to get Extra treasure in lower level dungeons for better low level gear. At least it will be fun and it will add a new feel to the game besides the grind. I think the only reason the 1st playthrough is fast is because of the 1st time bonus on completion and all the exp from quests.