I took the opportunity last night to do another test of the poncho liner under quilt. When I went to bed (11 PM), the temperature was around 42°F (4°C). I felt comfortable for about an hour, but I could feel cooling areas on my back. I had a small fleece blanket with me and I put that under my torso (shoulders to rump). I also used a down top quilt and slept the remaining night in bliss.
I'm a cold sleeper, so I wore a fleece top and bottom to bed. My legs were actually sweating in the morning.
According to my outdoor thermometer, the temperature dipped down to 37°F (3°C) around 6 AM when I got out of bed.
Below 40°F (4°C), I needed some additional insulation beyond the under quilt. The fleece throw worked great, actually, and was probably better (more breathable, stationary) than a CCF pad. I've been bringing this small fleece throw as a multi-use item on some trips. Normally, I roll it up and use it as a neck/head rest in the hammock.
My guess is that AS IS, the poncho liner under quilt is probably good down to 45°F (7°C) by itself. If you are expecting temperatures less than that, you'll want some additional insulation.
I ordered some Insultex insulation yesterday, which is suppose to be some high-tech miracle insulation. I'm planning on quilting a layer inside my poncho under quilt and sewing it up permanently. My hope is that this will make the poncho UQ a solid performer down to 35-40°F (2-4°C).
I think the cool areas were caused by gaps in the fabric, since the UQ is only loosely tied together.
I'm planning some canyoneering trips this summer where I'm expecting to get really wet (read: swimming), so I want to have some insulation that will perform well even in the odd chance it gets wet. I think this UQ will do the job!