"Or is the shell really of no consequence because the dew point is going to be somewhere inside the insulation anyways?
I think this is it. Even when I vent heavily, even mostly uncovered, I can still get condensation on the inside of my TiGoat bivy. That uses pretty thin material. When hiking I've had sweat vapor condense and freeze on the outside of my fleece. Both experiences make me think it doesn't really matter about the fabric breathability unless moist air is actively and forcefully pumped out.
Also, one big problem with being super breathable is that it means the slightest breeze can knock all the heat out of your quilt. You'd have to add a bivy to stop that problem, but then you lose the benefits of a super breathable shell.
A cuben quilt can definitely make you sweaty. The learning curve can be difficult, especially on humid nights in the high 40's, low 50's where it's cold enough to want to cover up, but warm enough that you need to vent a lot. I think that even with a bivy you can vent enough to stay pretty dry unless it's extremely humid. A few weeks ago I had a difficult night staying dry the whole night. I would do fine most of the time, but then a low cloud would settle into the valley and I'd instantly be soaked. Eventually the cloud would leave and I'd dry out, but I felt horrible for a while. Still, I doubt I would've done much better in a quilt made of traditional materials.
I do get sweaty in a synthetic sleeping bag too, so in a way I like the cuben quilt better. I still get sweaty, but I don't have to worry about the quilt getting wet, heavy and smelly. My clothes still get wet and smelly, but the wet part takes care of itself pretty quick once I start hiking. The smelly...well, I'm still figuring out how to do trail laundry well enough to kill the funk.