I've successfully used a no-sew poncho liner underquilt down to about 45, maybe a bit lower. (I added a layer of Insultex in the middle... it helped some)
My son (10) uses it when he hangs, but he could sleep naked in the snow almost... he's quite the space heater.
If you added say 2 layers of IX, and hung it right, you could get down to 40 if you're a warm sleeper... any lower and you'd need a pad or more insulation.
I did mine a little different, my poncho liner was missing most of the tie outs. I sewed grosgrain loops at the corners and at the fold point. I set it up that when I put the corner loops together, I pull the top loop through the bottom loop. This pulls the top loop tighter, leaving a really small gap between the layers of the UQ. Theoretically, this should make the UQ warmer. If you look at Dejoha's illustration: Instead of tying the 2 pieces of shock cord at the middle, I added a cordlock. That way, I could just pull the excess shock cord and the ends cinch up. Release the cord and I get my PL back to normal real quick.
Tonight, I'm hanging in the back yard, and my PL is acting as a cover over my new DL 1.0 BlackBird...