The class/job with the highest level gets no XP bonus.
classes/jobs, who are not your highest class job get a XP-boost of 50% and if they are below lvl 50 iirc, they get 100% XP-boost but only discipline of war and discipline of magic and only for killing XP, so not for quests.

To your question:
No, it wouldn't be detrimental.
You would secure the XP-Boost for your other classes/jobs until lvl 60 and shorten the needed dungeon runs by a good portion.

Also the main story quests are a very good XP ressource for your "main".