Ditto on what Derek said.
Under most situations you may not have a problem with the vapor passing though both layers. In high condensation conditions however the vapor won't make it all the way through in one shot(correct me if i am wrong) and this is when the problems will begin. If the vapor condenses to the underside of the m90 it will drip and then migrate to the areas of the bag that are cuben. Once the condensation vaporizes again (happens all night right, back and forth)it will not be able to pass out of the bag and then you are looking at the possibility of wetting out your down.
A better approach to make the bag more usable in warmer temps is to reduce the loft. I know that it makes the bag more specialized, but to be honest from the beginning we expected these to have a more narrow temp rating.
I think you could roll the dice and take the gamble and test the first momentum center cuben winged quilt, but how it will perform is very much theory vs proven fact.
If the ultimate in lightness is the goal go cuben/with m90 stripe and do sewn through with 1" loft. That quilt will go all summer (if you don't expect to bundle under it) and into fall spring with no issues. (where i hike *midwest*)
-Tim
