you need it to breathe. It's over your feet, they need the air. Once a friend of mine put the very foot of her bag in a garbage sack to help it stay dry, SOAKED the next day, yet no water in the tent. She couldn't figure it out. Your feet pump out moisture, i wouldn't risk it.
I used EPIC on a down quilt i made for ROG (maybe he'll comment on it) but i only sewed it up, i didn't get to use it. I have a bit left that i'll use for something i make this winter. But i don't know much about it yet. It is pretty light, only a bit heavier than 1.1sil. It was easy to work with and seemed to breathe well when i did the breathing test.