There should definitely be stat caps on content. The last time our FC brought some new players through the main story dungeons, they were ridiculously short and easy, and we let them watch all the cutscenes. Our overgeared characters with our stylish vanities completely trivialize any challenge they used to pose, which is detrimental to the atmosphere. This also applies to other dungeons: there should be a maintainable sense of challenge and threat.

The burden should not be on the players to keep lower-level gear just for this purpose, after all, where are we supposed to put all this stuff when we gear up 3-9 combat jobs?

As for echo buff on the harder content, I'm not adverse to this. I play Final Fantasy for the story, and in all the single player games, all the hard stuff is optional and contributes nothing to the story, but the story can be played through even by less skilled players. I think this fits the franchise quite well.