I have been playing with the same material and have met with similar results: the PEF rips easily, but perhaps a strip of clear packing tape along the sewing line could relieve this problem(?).
On the next trial I will definitely use 1.1 rs nylon on both sides for added strength. I also tossed a layer of Mylar space blanket in between the first and second layer for S&G's, and to see if I could squeeze a few more btu's out of the rig. I doesn't add any real weight to speak of.
On the issue of length, I have decided to stick with the 3/4 length. From a pack weight perspective and raw material dimensions, this just makes good practical sense (to me). I made a seat pad with the PEF that I can always put under my feet if necessary.
My TQ is made with the same material. I made a slot on the inside that allows me to insert additional layers as the weather requires. Great stuff that PEF!