This idea reminds me of the HH Super Shelter Sil-nyl under cover. Unlike rain falling from high in the sky and striking a sil-nyl tarp, rain blowing in from the ends or under the tarp sides is seldom at such a velocity, I am guessing. So I have never so far had anything at all get through. Of course, it does not cover the top, but still allows the use of a smaller tarp IMO. Add in the slightly water resistant top cover, and you have a pretty secure system against wind and wind driven rain and snow.
There is no concern of the fabric wetting thru and soaking your quilt. Of course, with the sil-nylon UC, you must take precautions against condensation soaking your quilt, i.e. a Heat Sheet or plain vapor barrier.