Got down to around 10* here this weekend...
Xtherm plus reflectix crapped out at around 12 I'd say. I was able to finish the night as the low was about 4-6am, but If I had started the evening closer to 10* I don't think it would have worked.

I have a 24" width of reflectix cut to match the large xtherm. I will try making a wider piece so it wraps the sides of the pad as the cold strips along the side were the most evident failure...
That said doing roughly 30" of reflectix will push that to 12-14 oz. So with the reflectix and large x therm I'm pushing 2lbs and it's getting a little silly.
A downmat would do the job in one piece for a little more if I wanted to be able to go to ground, and a 2lb UQ would be so warm that it's equally silly to ignore...

But for those pushing pads... the quest continues.