Coldest i've ever hung was 27F in Louisiana. This was done as an experiment to see what my rig could do. I was using a JRB original underquilt (rated to 35) and weathershield with a foam pad inside, and a 35F WM down bag. i wore long johns and a hat, and did fine. not cozy, but warm enough to sleep and wake up from the cold.
coldest i've ever slept out on the ground was in Jan 1992, at Fort Drum, NY. It was close to -40 at night for a couple weeks, -20 during the days... this, in an inadequate bag inside another inadequate bag, on top of 2 foam pads. it sucked massively.