I tried my Thermal Sheet just recently with temps down into the low 40's to 40. I was alone in a BA SL2, vestibule open w/ a breeze, footbox zipped to ~12", sleeping w/ nylon/poly pants, nylon L/S shirt, and UL merino socks. Supplemental insulating layer were close by..
But, no need for them.
I'm not known to be an unusually warm sleeper - with all of my WM bags seeming true to specs. I was only chilly when the breeze would blow on my head (not much more than 3/8" clipped hair). I did have an UL merino beanie that I wore once but I ended taking it off soon after donning.
All in all, great product, price and WEIGHT!
It is now my go-to summer bag/quilt here in the MT rockies...w/ my MB Exlight close by.