I guess by using a DWR bivy you eliminate the seams, so water will bead all the way off (not all of it, it will eventually wet through) much more easily. But yeah, I've never been totally sold myself on DWR bivies, thats why I don't own one. I've never had any trouble with wind; I position gear eg pack etc on the wind side or pitch tarp edge close to the ground.
Yep, Cuben will have condensation issues on the inside. I think I'm willing to give it a shot in a couple of years though; it will be humid but warm. Its also light! One thing I'm not sure about (need to search the forums...) is whether a down jacket underneath a VBL quilt is a good idea or not...I imagine the down will start to wet out. If thats the case, a Synthetic jacket is a better idea, which of course weighs more, but then it starts getting complicated as to whether or not the cuben shelled quilt is a good idea anymore or not...is it better then to have a DWR quilt that breathes, and invest another ounce or so somewhere on better shelter coverage?...which is getting into the realm of the tough question posted by the OP.
I guess the only real way to know these things is to bite the bullet and shell out some clams for all the gear and get some experience using it, then decide! So many $$$ though, ouch!