How difficult is it to run dungeons as classes, not jobs?

[Naberrie]

What makes me a bit more curious now is whether or not clever use of materia can close the gap, if only slightly.

There is a cap on materia. Pieces can only be maxed out via materia to an extent. You'll get a message when you try to afix materia that would go over the cap warning you.

I'm not saying it's impossible. But just that materia can only do so much. And considering that at that level there aren't many materia slots on each piece to work with, you certainly wouldn't be able to compensate for all the stats you'd be losing without the Crystal.