As some have wisely said, a good sleep mat...Down needs to loft in order to work well,
but you crush the bags down thats under you as you sleep....solution: -a down filled inflatable mat.

http://www.exped.com/usa/en/product-category/mats

Re; waterproof Gortex shells, are a double edged sword, they are less breathable than pertex so can more easily
contain/ trap perspiration moisture, reducing the efficiency of down...but even the most breathable shell materials
can’t entirely stop the dampening effect of such internal moisture passing across the surface of down. Capillary
action of water then works further against the down, allowing even the smallest amount of water to flow across
the fine textured face of a down feather quickly.

enter Water resistant down: - current technology allows down to be coated at the nano-molecular level making it
'hydrophobic' , the same principle of how a butterflies wings repels water.

ADDED: Theres no escaping the fact WP laminates slow the removal of anti-thermal efficiency perspiration contained
in a s/bag...but better to have reduced efficiency down, rather than be in situation where without a WP bag, you end
up with soggy totally useless down.

-Bulletproof and Waterproof don't mean Idiotproof.