I like this idea, with more risk comes greater rewards, or defeat.

I'm all for a "level - sync" option placed on the dungeon run, that would allow the party leader to set it or leave it be. However, once set, there would need to be penalties for turning it off, wouldn't want to run all the way to the boss and turn off the sync/cap.

More options for players the better. And I do think players would run it with the cap/sync set. When better gear, or HQ gear is offered up, acts as a good reward.

Adding a title/achievement, and even some special gear would be a good incentive for those interested to run the dungeon capped.